Boxify superslab registry, add bench profile, and document C7 hotpath experiments

docs/analysis/C7_FREE_HOTPATH.md

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+C7 Free Hotpath (design memo)
+=============================
+
+Goals
+-----
+- Flatten the dominant C7 free path to minimise branches and helper hops.
+- Keep safety checks boxed; keep hot lane minimal.
+
+Current typical path (C7)
+-------------------------
+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
+---------------
+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
+----------------
+- 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
+----------
+- Compare C7-only ops/s before/after.
+- Ensure remote/free-list invariants stay enforced in the slow path.