feat(parser): Phase 285A1.4 & A1.5 - Weak field sugar + Parser hang fix

A1.4: Add sugar syntax `public weak parent` ≡ `public { weak parent }`
A1.5: Fix parser hang on unsupported `param: Type` syntax

Key changes:
- A1.4: Extend visibility parser to handle weak modifier (fields.rs)
- A1.5: Shared helper `parse_param_name_list()` with progress-zero detection
- A1.5: Fix 6 vulnerable parameter parsing loops (methods, constructors, functions)
- Tests: Sugar syntax (OK/NG), parser hang (timeout-based)
- Docs: lifecycle.md, EBNF.md, phase-285a1-boxification.md

Additional changes:
- weak() builtin implementation (handlers/weak.rs)
- Leak tracking improvements (leak_tracker.rs)
- Documentation updates (lifecycle, types, memory-finalization, etc.)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docs/reference/environment-variables.md

@@ -251,6 +251,7 @@ NYASH_CLI_VERBOSE=2 \
 | `NYASH_GC_METRICS=1` | OFF | Any | GC メトリクス (text) |
 | `NYASH_GC_METRICS_JSON=1` | OFF | Any | GC メトリクス (JSON) |
 | `NYASH_VM_TRACE=1` | OFF | Any | VM 実行トレース |
+| `NYASH_LEAK_LOG={1\|2}` | OFF | Any | Exit-time leak report (Phase 285)。`1`=summary counts, `2`=verbose (names/entries) |
 
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