feat(stable-api): add RARRAY_AREF and RARRAY_ASET with write barrier

[ianks]

Summary

Batch 6 of the stable API implementation plan - Adds array access methods to the Stable API:

• RARRAY_AREF - Get element from array by index
• RARRAY_ASET - Set element in array by index (with GC write barrier)

⚠️ Draft Status: This PR depends on Batch 3 (PR #678) being merged first, as RARRAY_ASET requires RB_OBJ_WRITE for GC safety.

Changes

• stable_api.rs: Added rarray_aref() and rarray_aset() methods to StableApiDefinition trait
• Implementations: Version-specific implementations for all 7 Ruby versions (2.7, 3.0-3.4, 4.0)
  • RARRAY_AREF: Direct pointer access via rarray_const_ptr(obj).offset(idx)
  • RARRAY_ASET: Uses RB_OBJ_WRITE for GC safety - critical for correctness!
• Fallback support: C fallback in compiled.c and Rust wrapper in compiled.rs
• Public API: RARRAY_AREF and RARRAY_ASET macro wrappers in macros.rs
• Tests: Comprehensive parity tests in stable_api_test.rs

Testing

All tests pass successfully ✅

Dependencies

⚠️ Requires PR #678 (Batch 3: Write Barrier) to be merged first

Related Issues

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

Checklist

• Code follows project style guide
• All tests pass
• Documentation included
• No breaking changes
• Uses RB_OBJ_WRITE for GC safety