docs(physics): record cutover slice C2 completion

C2 landed at 63c601ff with dual review PASS after two fix rounds. The
plan records the halved allocation result and tightened gate, the
class-wide token-based staleness rework the pooling forced, the
documented residual floor (Core-side ~520 B/op deferred to the C3
activation gate as a possible C2b), and the two review maintenance
notes. Next slice: C3, the spawn-frequency host cutover of routes 1+8.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/plans/2026-08-02-placement-cutover.md

@@ -92,10 +92,28 @@ same commit) → docs/handoff commit. No workarounds; no fused slices.
   direct object-clock bypasses, and containing headless's uncaught
   prepared-collision `InvalidDataException` — IS the C3 route-1 flip and
   moves there. No separate C1 commit.
-- **C2 — placement allocation budget.** Pool or eliminate the operation/
-  projection envelope allocations on the accepted placement path (root
-  cause, not a raised cap), or obtain explicit user approval for a measured
-  budget. Re-measure; the regression gate keeps the ceiling.
+- **C2 — placement allocation budget — COMPLETE at `63c601ff`
+  (2026-08-02, dual reviews PASS after two fix rounds).** 2,032 → 944
+  B/op via pooled operation envelopes (bounded, reset-at-rent, double-
+  retire guarded, reset/dispose-cleared, ledger-visible), a cached
+  collision-callback delegate over an explicit context stack, and a
+  non-boxing pending-head read; gate tightened to 1,536. The pooling
+  forced a class-wide staleness rework: captured-token-vs-fresh-lookup at
+  every reentrancy-spanning frame (26-site audit), hoisted stack locals
+  for retail's handle_all_collisions bits, token-gated bookkeeping
+  writes, and a deliberately identity-agnostic settle path (retail's
+  SetPositionInternal completes unconditionally even for displaced
+  operations).
+  **Residual floor (documented at the gate, decision deferred to the C3
+  activation gate where the user is in the loop):** ~520 B/op inside
+  Core's `PhysicsEngine.SetPosition` (transition init / query-footprint
+  materialization — a potential C2b if C3's connected profile shows it
+  matters) and ~208 B/op of sorted-tree node per pending receipt.
+  **Maintenance notes from review (no action):** the no-reentrancy
+  proofs on the 15 surviving reference-based currency checks are
+  comment-enforced; `IsCurrent(Operation)` remains available and a new
+  reentrancy-spanning call site would silently inherit the tautology —
+  its doc comment warns.
 - **C3 — spawn-frequency cutover: routes 1 + 8.** Flip graphical AND
   headless initial Create/login registration to
   `RegisterEntityWithInitialResidence` + executor + placement receipts