Metal shader bridge skips draws: "Subgroup builtins are not available" #443

During a Rosetta-path title run (found while testing #442), the Metal shader bridge skipped draws with "Subgroup builtins are not available"; the skip counter passed 100000 within ~280 s, during UI asset loading.

Not caused by #442 (no shader conversion changes). Next steps: find which recompiler output uses subgroup builtins (wave ops / ballot / readlane), map them to MSL simdgroup functions (simd_ballot, simd_shuffle, simd_broadcast…) or a fallback, and add a shader-bridge test.

Possibly a major cause of missing UI/3D rendering. Refs #418.

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