[MLX][Spec] Add optional Gemma 4 target feedback

[wirybeaver]

Stack

PR 2 of 6. Depends on #1.

• Base: prototype/mlx-gemma4-mtp-verify-cache
• Head: prototype/mlx-gemma4-mtp-target-feedback

Scope

• Add an opt-in target adapter for exact logits, hidden rows, and assistant seeds.
• Integrate transactional two-query verification with the MLX runner.
• Preserve the target-only numerical path and native history/cache invariant.
• Pass pinned target revisions through MLX loading.

Target-only inference keeps its existing path unless feedback is requested. No assistant, speculative worker, or serving dispatch is added.

Tests

test/registered/unit/hardware_backend/mlx/test_gemma4_target_adapter.py

Result: 4 passed, 6 subtests passed.

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CI States

Latest PR Test (Base): ❌ Run #30053071879
Latest PR Test (Extra): ❌ Run #30053071712