perf(physics): reuse reset-complete transition scratch

Mirror retail's ten-deep LIFO transition lifetime, retain all query scratch with complete reset contracts, and remove Tier-0 enum boxing without changing collision decisions. Fresh and retained engines are bit-identical across the expanded oracle, while measured transition profiles now allocate 0 bytes per resolve.

docs/research/2026-07-25-slice-i1-transition-scratch.md

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+# Slice I1 — reset-complete transition and query scratch
+
+**Status:** COMPLETE
+**Parent:** `docs/plans/2026-07-25-modern-runtime-slice-i.md`
+**Scope:** storage lifetime and allocation only; retail collision math,
+comparisons, branch order, traversal order, and returned behavior are
+unchanged.
+
+## Retail lifetime carried forward
+
+Named retail establishes a retained, stack-disciplined transition lifetime:
+
+- `CTransition::makeTransition` `0x0050B150`
+- `CTransition::cleanupTransition` `0x00509DC0`
+- `CTransition::init` `0x00509DD0`
+- `COLLISIONINFO::init` `0x00509D60`
+- `SPHEREPATH::init` `0x0050C330`
+
+Retail owns ten pre-existing `CTransition` records, selects one by nesting
+depth, resets it before use, and releases it in LIFO order. The acdream port
+now gives each `PhysicsEngine` the same ten-deep, same-thread lease contract.
+`ResolveWithTransition` and `ResolvePlacement` always return their lease in a
+`finally` block.
+
+## Storage changes
+
+- `ObjectInfo`, `CollisionInfo`, `SpherePath`, and `Transition` have complete
+  `ResetForReuse` contracts.
+- Fixed sphere objects, collision GUID storage, exact-length walkable buffers,
+  candidate-cell storage, and reentrant ordered-cell lists retain identity.
+  Every logical value and retained buffer content is reset before reuse.
+- Neighboring-cell scans use a retained LIFO list arena. This is required
+  because a cell collision may recursively enter step-up/step-down and another
+  `CheckOtherCells` before the outer scan resumes.
+- `BSPQuery.CollisionSphere` is value/ref scratch instead of a short-lived
+  reference object.
+- Terrain's always-three-vertex surface sample is stored inline. Production
+  BSP entry points take center/radius values directly instead of constructing
+  temporary DAT `Sphere` objects.
+- `CellTransit` reuses the transition-owned ordered/deduplicated candidate
+  collection.
+- Walkable publication to `PhysicsBody` reuses only an exact-length backing
+  array; a real polygon-size change replaces it.
+- Hot enum predicates use equivalent bit masks. This removes Tier-0
+  `Enum.HasFlag` boxing while preserving the exact branch condition.
+
+The wider collision oracle exposed the reentrant-cell-list hazard before
+landing: three Facility Hub corridor seam replays initially threw
+`Collection was modified`. The final LIFO query arena keeps every nested
+scan independent; all three exact seam replays pass.
+
+## Differential and structural evidence
+
+`TransitionScratchResetTests`:
+
+- reflection-poisons every stored member;
+- resets and compares the complete logical graph with a fresh transition;
+- proves sphere, child, list, candidate, and walkable-buffer identities are
+  retained while their values are cleared;
+- proves exact-length walkable reuse and replacement on a true size change;
+- proves ten distinct nested leases, capacity rejection, LIFO enforcement,
+  same-thread enforcement, and clean reuse.
+
+`TransitionScratchDifferentialTests` runs a fresh-transition engine beside the
+production retained engine and compares every `ResolveResult` float by bits
+plus every public `PhysicsBody` side effect. The hostile alternating sequence
+covers:
+
+- successful and failed transitions;
+- grounded and airborne movement;
+- wall collision and carried sliding state;
+- step-up and step-down inputs;
+- two-sphere characters;
+- one-sphere projectiles;
+- viewer/camera sweeps;
+- placement overlap search;
+- different mover and designated-target identities.
+
+The fixed graph oracle, including corridor seams, cellar, doors, projectiles,
+camera, stairs, cylinder/sphere families, and the new I1 gates, is
+**118 passed / 0 skipped / 0 failed**.
+
+## Allocation result
+
+Release, one thread, 256 warmups and 4,096 measured resolves per profile:
+
+| Profile | I0 graph baseline | I1 retained scratch |
+|---|---:|---:|
+| player, two spheres | 4,448 B/resolve | **0 B/resolve** |
+| remote, two spheres | 4,448 B/resolve | **0 B/resolve** |
+| projectile, one 5 cm sphere | 6,848 B/resolve | **0 B/resolve** |
+| camera/viewer, one 30 cm sphere | 2,512 B/resolve | **0 B/resolve** |
+| grounded walkable publication | not isolated | **0 B/resolve** |
+
+This is an isolated transition/query-scratch gate. Optional diagnostics and
+capture remain intentionally allocating, and the ordinary connected
+frame-allocation profile is repeated at Slice I7 after flat-asset cutover.
+
+## Release gates
+
+- focused collision oracle: **118 passed**
+- complete Core tests: **3,250 passed / 2 skipped**
+- `dotnet build AcDream.slnx -c Release --no-restore`: **passed**
+- complete solution tests: **8,342 passed / 5 skipped**
+
+Issue #237 is closed. Slice I2 begins with immutable Core-only flat collision
+records and deterministic source-graph flattening; the current graph traversal
+remains the executable behavior oracle.