stale docs cleanup, semantic analyzer warning, and variable allocator refactoring

[cs01]

Summary

Shard D of the compiler rearchitecture plan: stale docs cleanup, semantic analyzer warning, and variable allocator refactoring.

• D1: Update stale docs — Removed the outdated "optional chaining compiles to direct access" warning from rules.md (generateOptionalChain already emits proper icmp/phi null checks). Updated the new in class field initializers note to reflect that codegen now handles it. Added regression test fixture for optional chaining on interfaces and strings.
• D2: Class field initializer warning — Added a compile-time warning in the semantic analyzer when new X() appears in a class field initializer, guiding developers to prefer constructor initialization.
• D3: Extract VarKind classification — Added a VarKind enum (27 variants) and classifyVariable() method to variable-allocator.ts, making the priority-ordered type classification logic explicit and independently testable.
• D4: Switch dispatch — Replaced the 54-line if/else chain in allocate() with a switch on VarKind, dispatching to the same existing allocateX() methods.

Files changed

• .claude/rules.md — removed stale warnings, updated class field initializer docs
• src/analysis/semantic-analyzer.ts — added new X() field initializer warning
• src/codegen/infrastructure/variable-allocator.ts — added VarKind enum, VarClassification interface, classifyVariable() method, switch dispatch
• tests/test-fixtures.ts — registered 2 new test fixtures
• tests/fixtures/types/optional-chaining.ts — new regression test
• tests/fixtures/classes/class-field-new.ts — new regression test

Test plan

• npm test — all tests pass (only pre-existing try-catch failures)
• bash scripts/self-hosting.sh — full 3-stage self-hosting passes
• New test fixtures compile and run correctly
• No new test failures introduced