feat(stable-api): add RB_OBJ_WRITE and RB_OBJ_WRITTEN for GC write barrier

[ianks]

Summary

Batch 3 of the stable API implementation plan - Adds GC write barrier methods RB_OBJ_WRITE and RB_OBJ_WRITTEN to the Stable API.

These methods are critical for maintaining GC correctness when storing VALUE references from one heap object to another. This is a prerequisite for Batch 6 (RARRAY_ASET).

⚠️ Draft Status: Ready for review but marking as draft to coordinate with other batches.

Changes

• stable_api.rs: Added rb_obj_write() and rb_obj_written() methods to StableApiDefinition trait
• Implementations: Version-specific implementations for all 7 Ruby versions (2.7, 3.0-3.4, 4.0)
  • Calls rb_gc_writebarrier() and rb_gc_writebarrier_unprotect() internally
  • Direct FFI bindings for better performance
• Fallback support: C fallback in compiled.c and Rust wrapper in compiled.rs
• Public API: RB_OBJ_WRITE and RB_OBJ_WRITTEN macro wrappers in macros.rs
• Tests: Comprehensive parity tests in stable_api_test.rs

Testing

All tests pass successfully ✅

Related Issues

• Implements Implement new StableApiDefinition GC methods #447
• Required for Batch 6: RARRAY_ASET (feat(stable-api): Add RARRAY_AREF and RARRAY_ASET for array element access #671)

Checklist

• Code follows project style guide
• All tests pass
• Documentation included
• No breaking changes