Opaque Garbage Collection Reference Manager API

Summary:
This PR implements a new, fully opaque API for external reference managers to participate in CPython’s garbage collection process, as described in PEP XXXX.

Motivation:

• Enables safe, cross-runtime GC (JPype, Pyodide, pythonnet, etc.)
• Provides a forward-compatible, contract-based extension point
• Does not expose or depend on GC internals

Key Features:

• Opaque, phase-based API (no GC internals exposed)
• Strict contract for extension authors
• Additive and opt-in: no impact if unused
• Fully documented and tested

Files Changed:

• Include/cpython/clgc.h: New public API
• Python/gc.c: Implementation and integration

Request for Review:

• API contract and extensibility
• Thread safety and error handling
• Suitability for known use cases (JPype, Pyodide)