feat(stable-api): Add RB_OBJ_PROMOTED for GC generation checking

[ianks]

Summary

Add stable API method for checking if a Ruby object has been promoted to the old generation in the GC, equivalent to RB_OBJ_PROMOTED.

Motivation

Understanding object GC generation is important for:

• Implementing efficient write barriers
• Debugging GC-related issues
• Performance analysis of object lifetimes
• Advanced GC-aware data structures

Ruby Source Reference

// include/ruby/internal/gc.h

static inline bool
RB_OBJ_PROMOTED_RAW(VALUE obj)
{
    RBIMPL_ASSERT_OR_ASSUME(RB_FL_ABLE(obj));
    return RB_FL_ANY_RAW(obj, RUBY_FL_PROMOTED);
}

static inline bool
RB_OBJ_PROMOTED(VALUE obj)
{
    if (! RB_FL_ABLE(obj)) {
        return false;
    }
    else {
        return RB_OBJ_PROMOTED_RAW(obj);
    }
}

Proposed API

/// Check if an object has been promoted to the old GC generation 
/// (akin to `RB_OBJ_PROMOTED`).
///
/// Objects start in the "young" generation and are promoted to "old"
/// after surviving one or more GC cycles. Old objects are scanned less
/// frequently, which is why write barriers are needed when storing
/// references from old to young objects.
///
/// # Safety
/// This function is unsafe because it may dereference a raw pointer
/// to access the object's flags. The caller must ensure the VALUE
/// is a valid Ruby object.
unsafe fn rb_obj_promoted(&self, obj: VALUE) -> bool;

Implementation Notes

• Check RUBY_FL_PROMOTED flag (same as RUBY_FL_WB_PROTECTED, value 1<<5)
• Must handle special constants (which are never promoted)
• Reference: include/ruby/internal/gc.h:705-734

Checklist

• Add method to StableApiDefinition trait
• Implement for each Ruby version
• Add C fallback in compiled.c
• Add public macro wrapper in macros.rs
• Add tests