migrate all tutorials from Markdown to Org mode

Converted with pandoc 3.7 (markdown → org), all 17 files:
- README, references, 15 tutorial units
- Internal file links updated from .md to .org
- Source code blocks (#+begin_src lean) preserved
- Tables, math notation, links intact

For the Emacs workflow.

README.org

@@ -0,0 +1,58 @@
+* Lean 4 + Programming Language Semantics
+:PROPERTIES:
+:CUSTOM_ID: lean-4-programming-language-semantics
+:END:
+A self-study curriculum for computer scientists who know PL theory and want
+to mechanize proofs in Lean 4 --- from syntax to soundness of the
+Hindley-Milner type system.
+
+** Structure
+:PROPERTIES:
+:CUSTOM_ID: structure
+:END:
+#+begin_example
+├── README.org                          ← this file
+├── tutorial-01-basics/                ← Lean 4 fundamentals
+│   ├── 01-types-and-functions.org
+│   ├── 02-inductive-types.org
+│   ├── 03-propositions-and-proofs.org
+│   ├── 04-quantifiers-and-equality.org
+│   ├── 05-advanced-tactics.org
+│   ├── 06-structures-type-classes.org
+│   └── 07-dependent-types.org
+├── tutorial-02-semantics/             ← PL semantics in Lean
+│   ├── 08-syntax-representation.org
+│   ├── 09-substitution.org
+│   ├── 10-small-step-semantics.org
+│   ├── 11-stlc-type-system.org
+│   ├── 12-type-safety.org
+│   ├── 13-hm-declarative.org
+│   ├── 14-algorithm-w.org
+│   └── 15-soundness-completeness.org
+└── references.org                      ← all cited resources
+#+end_example
+
+** How to Use
+:PROPERTIES:
+:CUSTOM_ID: how-to-use
+:END:
+1. Install Lean 4 (VS Code + =lean4= extension):
+   [[https://lean-lang.org/lean4/doc/quickstart.html][lean-lang.org/lean4/doc/quickstart.html]]
+2. Start with *Tutorial 1*, Unit 1. Each unit is a standalone =.org= file with
+   inline Lean exercises.
+3. Type the exercises into a =.lean= file and make it compile.
+4. After Tutorial 1, move to *Tutorial 2* to mechanize STLC and HM.
+
+** Prerequisites
+:PROPERTIES:
+:CUSTOM_ID: prerequisites
+:END:
+- Functional programming (Haskell/OCaml familiarity)
+- Basic PL theory (lambda calculus, operational semantics, type systems)
+- No prior proof assistant experience required
+
+** References
+:PROPERTIES:
+:CUSTOM_ID: references
+:END:
+See [[file:references.org]] for the full list of resources.