Document compiler contracts and refresh contributor guides

[cristianoc]

Stacked on #8597. Documentation only: no behavior change (full test battery and checkformat pass; the two expected-file changes track fixture comments that render inside error snippets).

Code-level contracts

• ctype.mli, btype.mli, subst.mli, types.mli: caller contracts for instantiation (instance, instance_poly ~fixed, instance_label), the object-field filters, enlargement, subtyping, copy sessions (with_copy_session, copy_type_desc), mutability cells, and for_saving independence.
• Implementation comments at the owning definitions: trail vs copy-session ownership and the two logging rules, Tsubst mark visibility across nested sessions vs per-session mutability duplication, the representative behavior of dup_mutability, the pre-tail-unification openness sample in unify_fields, the rewritten copy header (level rule, ?partial, ?env records-only), and the two coercion paths at the typecore call site.
• The polymorphic scheme discipline (use-sites instantiate; introduction sites type at a fixed instance and verify generality) documented at type_object_field_value / object_field_use_type and in the type-checker guide, including why type_label_exp's PR#4862 retry stays label-specific.

Guides and discoverability

• 新建 area guides: compiler/ml/README.md (type checker), compiler/syntax/README.md (parser/printer/JSX), compiler/core/README.md (Lambda/JS backend), rewatch/README.md, plus refreshed analysis/README.md and tools/README.md. AGENTS.md and CONTRIBUTING.md link every guide, so each maintained document has an incoming link from the repository entry path.
• Compiler-facing JSX transformation notes move to compiler/syntax/JSX.md; docs/JSXV4.md becomes a short pointer preserving established external links. docs/Syntax.md, docs/Formatter.md (moved), docs/TYPING.adoc, and docs/reactive_reanalyze_design.md (with its context7.json entry) are retired after auditing; stale scratch documents under compiler/core, compiler/ext, compiler/depends, and analysis/reanalyze are removed.
• Test comments that referenced the working design document are now self-contained.

🤖 Generated with Claude Code

https://claude.ai/code/session_01PCtQiaDijUqA2fujQXvKUw

[chatgpt-codex-connector]

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[cristianoc]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Can't wait for the next one!

Reviewed commit: 1176ff9b35

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• Open a pull request for review
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.33%. Comparing base (29c907b) to head (2a5d740).

Additional details and impacted files

@@              Coverage Diff               @@
##           object-stage0    #8598   +/-   ##
==============================================
  Coverage          76.33%   76.33%           
==============================================
  Files                476      476           
  Lines              63253    63253           
==============================================
  Hits               48284    48284           
  Misses             14969    14969           

Files with missing lines	Coverage Δ
analysis/reanalyze/src/dead_value.ml	85.58% <ø> (ø)
compiler/ml/btype.ml	88.59% <ø> (ø)
compiler/ml/ctype.ml	64.88% <100.00%> (ø)
compiler/ml/subst.ml	83.43% <ø> (ø)
compiler/ml/typecore.ml	86.07% <ø> (ø)

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commit: 2a5d740