C7 Free Hotpath (design memo)
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Goals
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- Flatten the dominant C7 free path to minimise branches and helper hops.
- Keep safety checks boxed; keep hot lane minimal.

Current typical path (C7)
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1. size→class LUT → `class_idx = 7`.
2. free gate / route box decides Tiny vs Pool.
3. Tiny free fast v2:
   - Policy/env checks,
   - TLS SLL push,
   - Warm/UC interaction as needed.
4. Multiple helper calls along the way (gate, policy, sll push).

Target hot lane
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1. Single policy snapshot for C7 (warm/page/tls on).
2. Straight to TLS SLL push with minimal bookkeeping.
3. Optional UC/Warm stats only in sampled mode.
4. Rare branches (remote/free-list edge cases) stay in boxed slow path.

Ideas to explore
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- Add `hak_tiny_free_fast_v2_c7()` inline used when `class_idx==7`.
- Fold gate/policy reads into one branch per free call.
- Keep TLS SLL push inline, push remote/cross-thread cases behind unlikely branches.

Validation
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- Compare C7-only ops/s before/after.
- Ensure remote/free-list invariants stay enforced in the slow path.