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.

references.org

@@ -0,0 +1,51 @@
+* References
+:PROPERTIES:
+:CUSTOM_ID: references
+:END:
+** Primary Learning Resources
+:PROPERTIES:
+:CUSTOM_ID: primary-learning-resources
+:END:
+| Abbr | Title                            | Link                                                        |
+|------+----------------------------------+-------------------------------------------------------------|
+| FPIL | /Functional Programming in Lean/ | https://lean-lang.org/functional_programming_in_lean/       |
+| TPIL | /Theorem Proving in Lean 4/      | https://leanprover.github.io/theorem_proving_in_lean4/      |
+| MiL  | /Mathematics in Lean/            | https://leanprover-community.github.io/mathematics_in_lean/ |
+
+** PL Semantics Formalizations in Lean 4
+:PROPERTIES:
+:CUSTOM_ID: pl-semantics-formalizations-in-lean-4
+:END:
+| Resource                | What it covers                                                        | Link                                       |
+|-------------------------+-----------------------------------------------------------------------+--------------------------------------------|
+| syndikos/lean4-stlc     | STLC: syntax, semantics, typing, metatheory (Progress + Preservation) | https://github.com/syndikos/lean4-stlc     |
+| chenson2018/LeanScratch | Untyped λ-calculus: de Bruijn + locally nameless, confluence proofs   | https://github.com/chenson2018/LeanScratch |
+| sdemos/type-inference   | STLC + HM type inference, soundness & completeness of W               | https://github.com/sdemos/type-inference   |
+| rami3l/PLFaLean         | PLFA Parts 2--3 translated to Lean 4                                  | https://github.com/rami3l/PLFaLean         |
+
+** PL Theory (Non-Lean)
+:PROPERTIES:
+:CUSTOM_ID: pl-theory-non-lean
+:END:
+| Resource                                                        | Format    | Link                                                                     |
+|-----------------------------------------------------------------+-----------+--------------------------------------------------------------------------|
+| Software Foundations Vol.2                                      | Rocq book | https://softwarefoundations.cis.upenn.edu/                               |
+| PLFA Part 2                                                     | Agda book | https://plfa.github.io/                                                  |
+| Chris Henson: Beginner Resources for Formalizing Lambda Calculi | Blog post | https://chrishenson.net/posts/2025-05-10-formalized_lambda_calculus.html |
+
+** HM Type System
+:PROPERTIES:
+:CUSTOM_ID: hm-type-system
+:END:
+| Resource                                                                   | Format     | Link                                                             |
+|----------------------------------------------------------------------------+------------+------------------------------------------------------------------|
+| Wikipedia: Hindley-Milner type system                                      | Reference  | https://en.wikipedia.org/wiki/Hindley%E2%80%93Milner_type_system |
+| Jeff Vaughan: "A proof of correctness for the HM type inference algorithm" | PDF (2008) | https://www.jeffvaughan.net/docs/hmproof.pdf                     |
+
+** Community
+:PROPERTIES:
+:CUSTOM_ID: community
+:END:
+- Official site: https://lean-lang.org/
+- Zulip chat: https://leanprover.zulipchat.com/
+- Lean 4 docs: https://lean-lang.org/documentation/