Qwen3.5 M4 gate reports success when required goldens are missing

[VikashLoomba]

Bug

tests/parity/test_qwen35_paged_engine.cpp promises that an unavailable gate exits 77 rather than reporting success, and CTest configures SKIP_RETURN_CODE 77. Three missing-artifact paths violate that contract:

• missing greedy_ids.npy returns normally;
• missing our_ids.npy returns normally;
• missing neartie_gap_mnats.npy returns normally.

A normal return lets doctest and CTest report success, producing a false-green correctness gate. The immutable PR #559 head contains every artifact and the real ROCm gate passes, so this appears only when a committed golden is absent or a checkout is incomplete.

Required repair

• Route every unavailable prerequisite through the existing exit-77 skip behavior.
• Add a deterministic test that removes or redirects each required artifact and proves process exit 77, without requiring GPU hardware.
• Preserve hard failure for malformed or inconsistent artifacts; only unavailable prerequisites skip.
• Record RED, GREEN, and mutation evidence in the owning M4 spec.

Ownership

Owned and fixed in-flow by BACKEND-ROCM, PR #559, during the final current-main review.

[VikashLoomba]

Fixed in reviewed PR #559 at immutable final head 4673a35dd47087cae3f6f9808e4fcf64667bcbcf. Fresh mutation review proves missing-artifact exits 77/77/77, complete-probe hard exit 86, malformed-array failure, bootstrap skip, real 16/16 ROCm execution, and full preflight.