test(jpeg-metal): record blocked restart validation optimization

[jcwal1516]

Task 11 remains BLOCKED: removing CPU entropy traversal cannot preserve the completed decode validation contract with the current Metal status checks. No production shortcut is enabled.

This diagnostic creates a valid restart JPEG and removes its first entropy interval while retaining marker structure. The existing packet builder rejects it with a typed Huffman error. A privately modified test packet modeling marker-only preparation is accepted by full-image GPU status checks while producing different pixels; a final-MCU ROI also succeeds because the malformed interval is unselected. Production continues to use the fully validated packet builder. This test is a blocker reproduction, not a claim that the optimization has been implemented.

Based directly on main; the rejected task 01 refill candidate is not selected. Validation on local Apple M4 Pro: the named test ran with J2K_REQUIRE_METAL_RUNTIME=1, one passed and none skipped; package formatting and diff checks passed. Clippy tests passed with the repository's existing non-production allocation-lint exceptions. Initial strict test Clippy exposed those allocation lints and a long test body; duplicate decode setup was consolidated and the final hardware test rerun successfully.

Performance: NOT MEASURED, because no production optimization was selected. Remaining requirements for any future shortcut include segment-local bounds, entropy and terminal validation, error parity, full-image and partial-route parity, shared-consumer/CUDA validation, and the exact-head manual GPU workflow before merge. No manual workflow was dispatched. CUDA remains deferred to the end as requested.