feat(stable-api): add accessor methods (RSTRING_END, RDATA_PTR, RB_OBJ_FREEZE, RB_OBJ_PROMOTED)

[ianks]

Summary

Batch 5 of the stable API implementation plan - Adds simple accessor methods to the Stable API:

• RSTRING_END - Get pointer to end of string contents
• RDATA_PTR - Get data pointer from RData object
• RB_OBJ_FREEZE - Freeze an object
• RB_OBJ_PROMOTED - Check if object is promoted to old GC generation

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

Changes

• stable_api.rs: Added rstring_end(), rdata_ptr(), rb_obj_freeze(), rb_obj_promoted(), and rb_obj_promoted_raw() methods
• Implementations: Version-specific implementations for all 7 Ruby versions (2.7, 3.0-3.4, 4.0)
  • RSTRING_END: Composed from rstring_ptr + rstring_len
  • RDATA_PTR: Direct access to RData.data field
  • RB_OBJ_FREEZE: Calls rb_obj_freeze_inline()
  • RB_OBJ_PROMOTED: Checks RUBY_FL_PROMOTED flag
• Fallback support: C fallback in compiled.c and Rust wrapper in compiled.rs
• Public API: Macro wrappers in macros.rs with comprehensive documentation
• Tests: Parity tests in stable_api_test.rs

Testing

All tests pass successfully ✅

Related Issues

• Implements feat(stable-api): Add RSTRING_END for string end pointer #670 (RSTRING_END)
• Implements feat(stable-api): Add RDATA_PTR for legacy wrapped data access #666 (RDATA_PTR)
• Implements feat(stable-api): Add RB_OBJ_FREEZE for object freezing #672 (RB_OBJ_FREEZE)
• Implements feat(stable-api): Add RB_OBJ_PROMOTED for GC generation checking #673 (RB_OBJ_PROMOTED)

Checklist

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