fix(rewrite): toString normalization to BoxCall(slot #0) - Phase 287 P4

Root cause: toString/stringify/str were being rewritten to Global/Method calls
with class inference, causing Main.toString/0 to be called for primitives.

Fix (Box-First + Legacy Deletion):
1. ✅ MIR Builder - toString normalization (special.rs)
   - ALWAYS emit BoxCall with method_id=0 for toString/stringify/str
   - Do NOT rewrite to Global(Class.str/0) or Method calls
   - DELETED 70+ lines of complex class inference logic
   - Primitive guard with method name filter (known.rs)

2. ✅ JSON Serializer - method_id output (mir_json_emit.rs)
   - Include method_id field in BoxCall JSON for LLVM

3. ✅ LLVM Backend - universal slot #0 support
   - Extract method_id from JSON (instruction_lower.py)
   - Box primitives via nyash.box.from_i64 (boxcall.py)
   - Invoke toString via plugin system with method_id=0
   - ⚠️ TODO: Add nyash.integer.tostring_h to kernel

Test Results:
✅ VM: local x = 1; print(x.toString()) → "1" (PASS)
✅ VM: array_length test (boxed Integer) → PASS
⚠️ LLVM: Compiles successfully, needs kernel function

SSOT: slot_registry - toString is ALWAYS universal slot #0

Legacy Deleted:
- special.rs: Complex class inference rewrite (~70 lines)
- special.rs: Unique suffix fallback for toString
- special.rs: Main box special handling

Files changed:
- src/mir/builder/rewrite/special.rs (try_early_str_like_to_dst)
- src/mir/builder/rewrite/known.rs (primitive guards x4)
- src/runner/mir_json_emit.rs (method_id serialization x2)
- src/llvm_py/builders/instruction_lower.py (method_id extraction)
- src/llvm_py/instructions/boxcall.py (slot #0 handler)
- docs/reference/language/quick-reference.md (toString SSOT)

🎊 Generated with Claude Code
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docs/reference/language/README.md

@@ -19,6 +19,7 @@ Imports and namespaces
 Variables and scope
 - See: reference/language/variables-and-scope.md — Block-scoped locals, assignment resolution, and strong/weak reference guidance.
 - See: reference/language/lifecycle.md — Box lifetime, ownership (strong/weak), and finalization (`fini`) SSOT.
+- See: reference/language/repl.md — REPL mode semantics (file mode vs REPL binding rules).
 
 Type system (SSOT)
 - See: reference/language/types.md — runtime truthiness, `+`/compare/equality semantics, and the role/limits of MIR type facts.