Allow switching rendering backend without restarting the app

The renderer setting previously forced an app quit (exit(0)) because
switching backends between sessions crashed or leaked. Fix the
underlying lifecycle bugs and remove the restart requirement:

- FrameQueue: really release frames in clear(), clear on session
  start/stop, and guard start/stop with an owner so a stale session's
  async cleanup can't pause a new session's queue
- MetalView: weak delegate (breaks retain cycle leaking the whole Metal
  stack), never start two render threads, bound the shutdown wait to
  avoid deadlocking against the render thread's dispatch_sync to main
- MetalViewController: explicit idempotent shutdown called from stream
  teardown (viewDidDisappear isn't guaranteed); snapshot the renderer
  in render-thread callbacks to avoid use-after-free during shutdown
- MetalVideoRenderer: make the shared colorspace name thread-safe and
  ARC-managed instead of a CFStringRef over-released in dealloc; guard
  NULL format description extensions
- Connection: drop the static renderer reference in DrCleanup
- VideoDecoderRenderer: cache the application background state via
  notifications instead of calling UIKit from the decode queue
  (Main Thread Checker violation)
- Frame: don't CFRetain(NULL) when a format desc has no extensions
- StreamFrameViewController: fix _streamView cleanup ordering
- Settings: apply the backend change in place (with the Metal caveats
  alert) and update localized tips; takes effect on next stream

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

VoidLink/Localization/Localizable.xcstrings

@@ -5090,6 +5090,29 @@
         }
       }
     },
+    "Apply" : {
+      "extractionState" : "stale",
+      "localizations" : {
+        "zh-Hans" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "应用"
+          }
+        },
+        "zh-Hant" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "套用"
+          }
+        },
+        "zh-HK" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "套用"
+          }
+        }
+      }
+    },
     "Arrow direction pad" : {
       "extractionState" : "manual",
       "localizations" : {
@@ -8208,6 +8231,29 @@
         }
       }
     },
+    "Enable Metal Renderer?" : {
+      "extractionState" : "stale",
+      "localizations" : {
+        "zh-Hans" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "启用 Metal 渲染？"
+          }
+        },
+        "zh-Hant" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "啟用 Metal 渲染？"
+          }
+        },
+        "zh-HK" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "啟用 Metal 渲染？"
+          }
+        }
+      }
+    },
     "enableOswForNativeTouchSwitchTip" : {
       "extractionState" : "manual",
       "localizations" : {
@@ -9304,98 +9350,6 @@
         }
       }
     },
-    "Interpolation failed for an unknown reason." : {
-      "extractionState" : "manual",
-      "localizations" : {
-        "zh-Hans" : {
-          "stringUnit" : {
-            "state" : "translated",
-            "value" : "未知原因导致的插帧失败"
-          }
-        },
-        "zh-Hant" : {
-          "stringUnit" : {
-            "state" : "translated",
-            "value" : "未知原因導致插幀失敗"
-          }
-        },
-        "zh-HK" : {
-          "stringUnit" : {
-            "state" : "translated",
-            "value" : "未知原因導致插幀失敗"
-          }
-        }
-      }
-    },
-    "Interpolation overhead is too high. Falling back to normal mode." : {
-      "extractionState" : "manual",
-      "localizations" : {
-        "zh-Hans" : {
-          "stringUnit" : {
-            "state" : "translated",
-            "value" : "插帧性能开销过大，回退普通模式"
-          }
-        },
-        "zh-Hant" : {
-          "stringUnit" : {
-            "state" : "translated",
-            "value" : "插幀效能開銷過大，回退至一般模式"
-          }
-        },
-        "zh-HK" : {
-          "stringUnit" : {
-            "state" : "translated",
-            "value" : "插幀效能開銷過大，回退至一般模式"
-          }
-        }
-      }
-    },
-    "Interpolator error:" : {
-      "extractionState" : "manual",
-      "localizations" : {
-        "zh-Hans" : {
-          "stringUnit" : {
-            "state" : "translated",
-            "value" : "插帧器错误："
-          }
-        },
-        "zh-Hant" : {
-          "stringUnit" : {
-            "state" : "translated",
-            "value" : "插幀器錯誤："
-          }
-        },
-        "zh-HK" : {
-          "stringUnit" : {
-            "state" : "translated",
-            "value" : "插幀器錯誤："
-          }
-        }
-      }
-    },
-    "Interpolator started." : {
-      "extractionState" : "manual",
-      "localizations" : {
-        "zh-Hans" : {
-          "stringUnit" : {
-            "state" : "translated",
-            "value" : "插帧器已启动。"
-          }
-        },
-        "zh-Hant" : {
-          "stringUnit" : {
-            "state" : "translated",
-            "value" : "插幀器已啟動。"
-          }
-        },
-        "zh-HK" : {
-          "stringUnit" : {
-            "state" : "translated",
-            "value" : "插幀器已啟動。"
-          }
-        }
-      }
-    },
     "framePacingStackTip" : {
       "extractionState" : "manual",
       "localizations" : {
@@ -10883,6 +10837,98 @@
         }
       }
     },
+    "Interpolation failed for an unknown reason." : {
+      "extractionState" : "manual",
+      "localizations" : {
+        "zh-Hans" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "未知原因导致的插帧失败"
+          }
+        },
+        "zh-Hant" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "未知原因導致插幀失敗"
+          }
+        },
+        "zh-HK" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "未知原因導致插幀失敗"
+          }
+        }
+      }
+    },
+    "Interpolation overhead is too high. Falling back to normal mode." : {
+      "extractionState" : "manual",
+      "localizations" : {
+        "zh-Hans" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "插帧性能开销过大，回退普通模式"
+          }
+        },
+        "zh-Hant" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "插幀效能開銷過大，回退至一般模式"
+          }
+        },
+        "zh-HK" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "插幀效能開銷過大，回退至一般模式"
+          }
+        }
+      }
+    },
+    "Interpolator error:" : {
+      "extractionState" : "manual",
+      "localizations" : {
+        "zh-Hans" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "插帧器错误："
+          }
+        },
+        "zh-Hant" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "插幀器錯誤："
+          }
+        },
+        "zh-HK" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "插幀器錯誤："
+          }
+        }
+      }
+    },
+    "Interpolator started." : {
+      "extractionState" : "manual",
+      "localizations" : {
+        "zh-Hans" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "插帧器已启动。"
+          }
+        },
+        "zh-Hant" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "插幀器已啟動。"
+          }
+        },
+        "zh-HK" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "插幀器已啟動。"
+          }
+        }
+      }
+    },
     "Interval of Multi-Touch Move Events: %d μs" : {
       "extractionState" : "manual",
       "localizations" : {
@@ -12193,25 +12239,25 @@
         "en" : {
           "stringUnit" : {
             "state" : "translated",
-            "value" : "\nHold up! \n\n- Standard render has better performance on most devices now.\n\n- Changing the rendering mode requires restarting the application to ensure a clean start.\n\n- Metal rendering is an experimental feature that may cause performance degradation on some devices.\n\n- Metal render currently is unable to handle HDR content correctly on most devices.\n\n- Metal rendering mode does not support PiP.\n\n- Tapping “Quit Now” means I acknowledge the above information."
+            "value" : "\nHold up! \n\n- Standard render has better performance on most devices now.\n\n- The change takes effect the next time you start a stream. No app restart is needed.\n\n- Metal rendering is an experimental feature that may cause performance degradation on some devices.\n\n- Metal render currently is unable to handle HDR content correctly on most devices.\n\n- Metal rendering mode does not support PiP.\n\n- Tapping “Apply” means I acknowledge the above information."
           }
         },
         "zh-Hans" : {
           "stringUnit" : {
             "state" : "translated",
-            "value" : "\n等等！\n\n- 标准渲染模式目前在大多数设备上具有更好的性能。\n\n- 切换渲染模式需要重启应用程序，以确保一次干净的启动。\n\n- Metal 渲染是一项实验性功能，在某些设备上可能会导致性能下降。\n\n- Metal 渲染模式下不支持画中画。\n\n- 目前 Metal 渲染器在大多数设备上无法正确处理 HDR 内容。\n\n点击“立即退出”表示我已知晓并认可以上信息。"
+            "value" : "\n等等！\n\n- 标准渲染模式目前在大多数设备上具有更好的性能。\n\n- 渲染模式的更改将在下次开始串流时生效，无需重启应用程序。\n\n- Metal 渲染是一项实验性功能，在某些设备上可能会导致性能下降。\n\n- Metal 渲染模式下不支持画中画。\n\n- 目前 Metal 渲染器在大多数设备上无法正确处理 HDR 内容。\n\n点击“应用”表示我已知晓并认可以上信息。"
           }
         },
         "zh-Hant" : {
           "stringUnit" : {
             "state" : "translated",
-            "value" : "\n等等！\n\n- 目前在大多数装置上，標準渲染模式具有较佳的效能。\n\n- 变更渲染模式需要重新启动应用程式，以确保干净的启动流程。\n\n- Metal 渲染为实验性功能，可能在某些装置上造成效能下降。\n\n- Metal 渲染模式下不支援画中画。\n\n- Metal 渲染器目前在大多数装置上无法正确处理 HDR 内容。\n\n点选「立即结束」表示我已了解并同意上述说明。"
+            "value" : "\n等等！\n\n- 目前在大多数装置上，標準渲染模式具有较佳的效能。\n\n- 变更渲染模式将在下次开始串流时生效，无需重新启动应用程式。\n\n- Metal 渲染为实验性功能，可能在某些装置上造成效能下降。\n\n- Metal 渲染模式下不支援画中画。\n\n- Metal 渲染器目前在大多数装置上无法正确处理 HDR 内容。\n\n点选「套用」表示我已了解并同意上述说明。"
           }
         },
         "zh-HK" : {
           "stringUnit" : {
             "state" : "translated",
-            "value" : "\n等等！\n\n- 目前在大多数装置上，標準渲染模式具有较佳的效能。\n\n- 变更渲染模式需要重新启动应用程式，以确保干净的启动流程。\n\n- Metal 渲染为实验性功能，可能在某些装置上造成效能下降。\n\n- Metal 渲染模式下不支援画中画。\n\n- Metal 渲染器目前在大多数装置上无法正确处理 HDR 内容。\n\n点选「立即结束」表示我已了解并同意上述说明。"
+            "value" : "\n等等！\n\n- 目前在大多数装置上，標準渲染模式具有较佳的效能。\n\n- 变更渲染模式将在下次开始串流时生效，无需重新启动应用程式。\n\n- Metal 渲染为实验性功能，可能在某些装置上造成效能下降。\n\n- Metal 渲染模式下不支援画中画。\n\n- Metal 渲染器目前在大多数装置上无法正确处理 HDR 内容。\n\n点选「套用」表示我已了解并同意上述说明。"
           }
         }
       }
@@ -15619,25 +15665,25 @@
         "en" : {
           "stringUnit" : {
             "state" : "translated",
-            "value" : "- Standard render has better performance on most devices now.\n\n- Changing the rendering mode requires restarting the application to ensure a clean start.\n\n- Metal rendering is an experimental feature that may cause performance degradation on some devices.\n\n- Metal render currently is unable to handle HDR content correctly on most devices.\n\n- Metal rendering mode does not support PiP."
+            "value" : "- Standard render has better performance on most devices now.\n\n- Changing the rendering mode takes effect the next time you start a stream.\n\n- Metal rendering is an experimental feature that may cause performance degradation on some devices.\n\n- Metal render currently is unable to handle HDR content correctly on most devices.\n\n- Metal rendering mode does not support PiP."
           }
         },
         "zh-Hans" : {
           "stringUnit" : {
             "state" : "translated",
-            "value" : "- 目前, 标准渲染模式在大多数设备上具有更好的性能。\n\n- 切换渲染模式需要重启应用程序，以确保一次干净的启动。\n\n- Metal 渲染是一项实验性功能，在某些设备上可能会导致性能下降。\n\n- Metal 渲染模式下不支持画中画。\n\n- 目前 Metal 渲染器在大多数设备上无法正确处理 HDR 内容。\n"
+            "value" : "- 目前, 标准渲染模式在大多数设备上具有更好的性能。\n\n- 渲染模式的更改将在下次开始串流时生效。\n\n- Metal 渲染是一项实验性功能，在某些设备上可能会导致性能下降。\n\n- Metal 渲染模式下不支持画中画。\n\n- 目前 Metal 渲染器在大多数设备上无法正确处理 HDR 内容。"
           }
         },
         "zh-Hant" : {
           "stringUnit" : {
             "state" : "translated",
-            "value" : "- 目前，在大多数装置上，標準渲染模式具有较佳的效能。\n\n- 变更渲染模式需要重新启动应用程式，以确保干净的启动流程。\n\n- Metal 渲染为实验性功能，可能在某些装置上造成效能下降。\n\n- Metal 渲染模式下不支援画中画。\n\n- Metal 渲染器目前在大多数装置上无法正确处理 HDR 内容。"
+            "value" : "- 目前，在大多数装置上，標準渲染模式具有较佳的效能。\n\n- 变更渲染模式将在下次开始串流时生效。\n\n- Metal 渲染为实验性功能，可能在某些装置上造成效能下降。\n\n- Metal 渲染模式下不支援画中画。\n\n- Metal 渲染器目前在大多数装置上无法正确处理 HDR 内容。"
           }
         },
         "zh-HK" : {
           "stringUnit" : {
             "state" : "translated",
-            "value" : "- 目前，在大多数装置上，標準渲染模式具有较佳的效能。\n\n- 变更渲染模式需要重新启动应用程式，以确保干净的启动流程。\n\n- Metal 渲染为实验性功能，可能在某些装置上造成效能下降。\n\n- Metal 渲染模式下不支援画中画。\n\n- Metal 渲染器目前在大多数装置上无法正确处理 HDR 内容。"
+            "value" : "- 目前，在大多数装置上，標準渲染模式具有较佳的效能。\n\n- 变更渲染模式将在下次开始串流时生效。\n\n- Metal 渲染为实验性功能，可能在某些装置上造成效能下降。\n\n- Metal 渲染模式下不支援画中画。\n\n- Metal 渲染器目前在大多数装置上无法正确处理 HDR 内容。"
           }
         }
       }
@@ -17550,25 +17596,25 @@
         "en" : {
           "stringUnit" : {
             "state" : "translated",
-            "value" : "\n- Standard render has better performance on most devices now.\n\n- Changing the rendering mode requires restarting the application to ensure a clean start."
+            "value" : "\n- Standard render has better performance on most devices now.\n\n- The change takes effect the next time you start a stream."
           }
         },
         "zh-Hans" : {
           "stringUnit" : {
             "state" : "translated",
-            "value" : "\n- 标准渲染模式目前在大多数设备上具有更好的性能。\n\n- 更改渲染模式需要重启应用以确保干净启动。您是否要立即退出应用？"
+            "value" : "\n- 目前, 标准渲染模式在大多数设备上具有更好的性能。\n\n- 渲染模式的更改将在下次开始串流时生效。"
           }
         },
         "zh-Hant" : {
           "stringUnit" : {
             "state" : "translated",
-            "value" : "\n- 目前在大多数装置上，標準渲染模式具有较佳的效能。\n\n- 更改渲染模式需要重新啟動應用程式以確保乾淨啟動。您是否要立即退出應用程式？"
+            "value" : "\n- 目前，在大多数装置上，標準渲染模式具有较佳的效能。\n\n- 变更渲染模式将在下次开始串流时生效。"
           }
         },
         "zh-HK" : {
           "stringUnit" : {
             "state" : "translated",
-            "value" : "\n- 目前在大多数装置上，標準渲染模式具有较佳的效能。\n\n- 更改渲染模式需要重新啟動應用程式以確保乾淨啟動。您是否要立即退出應用程式？"
+            "value" : "\n- 目前，在大多数装置上，標準渲染模式具有较佳的效能。\n\n- 变更渲染模式将在下次开始串流时生效。"
           }
         }
       }
@@ -19915,4 +19961,4 @@
     }
   },
   "version" : "1.1"
-}
+}