[MLX][Spec] Add synchronous Gemma 4 MTP worker

[wirybeaver]

Stack

PR 4 of 6. Depends on #3.

• Base: prototype/mlx-gemma4-mtp-assistant
• Head: prototype/mlx-gemma4-mtp-worker

Scope

Adds the dormant MLX-native, one-draft synchronous worker:

• atomic verify → prepare candidate → propose → commit orchestration;
• CPU-only spec-v2 relay and scheduler-owned commit accounting;
• finish, failure, and cache-clear lifecycle cleanup;
• CPU request-slot assignment without Triton.

Serving dispatch is intentionally deferred to the next PR. Tests use one tiny target and a scripted proposer, with no checkpoint or broad matrix.

Tests

test/registered/unit/hardware_backend/mlx/test_gemma4_mtp_worker.py

Covers accept/reject parity beyond cache rotation, scheduler commit ownership, proposal-failure rollback and retry, natural-finish cleanup, and CPU slot fallback. Result: 4 passed, 4 subtests passed.

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CI States

Latest PR Test (Base): ❌ Run #30053099858
Latest PR Test (Extra): ❌ Run #30053099701