[Bug] Fix SPIR-V dominance of the adstack heap stride loads.

[duburcqa]

Issue: #805

The reverse-mode collapse reported in #805 is not an Apple compiler uniformity bug: get_ad_stack_metadata_stride_(float|int) memoize the stride OpLoad at their first call site, so when the first f32 heap access of a task sits inside one arm of a runtime if, every access in the sibling arm reuses an SSA id that does not dominate it. spirv-val rejects the module (section 2.16), but SPIRV-Cross does not validate and emits the reused id as a plain uninitialized MSL variable, so the per-thread heap base row_id * stride multiplies a correct per-lane row_id by a garbage stride (typically 0) and every thread of the dispatch races on the same heap row. The fix seeds both memoized stride loads at the task entry block (preload_ad_stack_metadata_strides, called from each generate_*_kernel, gated on the pre-pass adstack count so adstack-free tasks do not bind the metadata buffer).

Brief Summary

copilot:summary

Walkthrough

copilot:walkthrough

[duburcqa]

Genesis PR #2842 (29e2c22e5) x quadrants PR #806 (fe617bffe), with the Genesis-side Metal gradient skip lifted.

MacBook Pro: pytest tests/grad -n 8 --backend cpu

================= 58 passed, 12 warnings in 233.71s (0:03:53) ==================

MacBook Pro: pytest tests/grad -n 8 --backend gpu

============ 56 passed, 2 skipped, 12 warnings in 242.28s (0:04:02) ============

The 2 skips are the fp64 arms, which Metal does not support.

[hughperkins]

@codex review

[hughperkins]

• skimming the code looks reasonable to me: relatively small well-defined changes, albeit to general spirv file ✅
• lets wait for CI agent checks to run ⌛

[chatgpt-codex-connector]

Codex Review: Something went wrong. Try again later by commenting “@codex review”.

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[github-actions]

Total: 3 file(s) changed, +41 -0 code lines.

[hughperkins]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep them coming!

Reviewed commit: fe617bffe8

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[duburcqa]

Production CI: GS_ENABLE_NDARRAY=1 pytest -v -ra --dev --backend gpu --forked tests/grad
============== 58 passed, 17 warnings in 278.54s (0:04:38) ===========

[github-actions]

Diff coverage: 70% · 234 lines, 70 missing

[hughperkins]

ok to merge

[hughperkins]

Note: no need for Genesis benchmarks, because spir-v specific, and benchmarks don't exercise spir-v.