[MLX][Spec] Add one-draft verification and cache transactions

[wirybeaver]

Stack

PR 1 of 6.

• Base: feature/mlx-gemma4-native-cache
• Head: prototype/mlx-gemma4-mtp-verify-cache

Scope

• Add pure, lossless greedy verification for one assistant draft.
• Add clone/replay native-cache transactions with atomic commit and abort.
• Cover acceptance, rejection, rotating caches, and replay failures.
• Consolidate the tiny Gemma fixtures in the registry-exempt utils.py helper.

This PR adds no assistant, worker, scheduler dispatch, or CLI enablement; serving remains dormant.

Tests

test_gemma4_native_cache.py, test_gemma4_cache_transaction.py, and test_mlx_spec_decode.py

Result: 11 passed, 24 subtests passed. The registered-test CI checker also passes.

---

CI States

Latest PR Test (Base): ❌ Run #30053058577
Latest PR Test (Extra): ❌ Run #30053058474