ship(phys): A6.P3 slice 1 — Indoor ContactPlane retention COMPLETE

Slice 1 ships the strip-synthesis + Mechanism B (LKCP restore) fix
addressing A6.P2 Finding 2. Includes:

  - scen3_inn_2nd_floor_postfix paired capture (retail.log + decoded
    + acdream.log) as verification artifact. acdream cp-write count
    dropped from 86,748 to 25,082; per-unit-of-activity rate dropped
    63x (164.61 -> 2.60 cp-writes per cell-cache event).

  - Findings doc (docs/research/2026-05-21-a6-cdb-capture-findings.md)
    appended with slice 1 SHIPPED section: commit map, scen2/scen3
    pre/post comparison tables, user happy-test results, status of
    each A6.P2 finding (Finding 1 CLOSED as side-effect, Finding 2
    PARTIALLY CLOSED with remaining 99.3% from L622 seed flagged
    as #96, Finding 3 + #95 still open), slice 2 recommendation.

  - Issue #96 filed: "Per-tick PhysicsEngine.ResolveWithTransition CP
    seed contributes 99.3% of post-slice-1 CP writes." Slice 2 target.
    Sketch options: remove entirely / gate by change-detection / no-op
    guard inside SetContactPlane.

  - Issue #97 filed: "Phantom collisions + occasional fall-through on
    indoor 2nd floor." User-reported during happy-testing. HYPOTHESIS:
    side-effect of #96; falsifiable by re-testing post-slice-2.

  - CLAUDE.md updated: Currently-working-toward block now points at
    A6.P3 slice 2 (#96) as the active phase. M1.5 building/cellar
    demo half is ACHIEVABLE NOW (slice 1 unblocked the physics).

  - Roadmap updated: A6.P3 broken into 3 slices, slice 1 marked
    SHIPPED with commit history.

KEY USER-VISIBLE OUTCOME: stairs + cellar descent now WORK in acdream
(user happy-test confirmed walking up/down inn stairs multiple times,
walking down to cellar). A6.P2 Finding 1 (dispatcher entry frequency
mismatch) confirmed as secondary effect of Finding 2 — closed without
explicit Finding 1 work, as A6.P2 hypothesized.

REMAINING CONCERNS for slice 2 + future:
  - L622 per-tick seed (#96) still firing 24,906 times in scen3 postfix
  - Phantom collisions + fall-through on 2nd floor (#97)
  - See-through-walls indoors (#95, separate scope)
  - Indoor lighting broken (A7 scope)

Test suite: 1148 pass + 8 pre-existing fail (baseline maintained;
T3 IndoorContactPlaneRetentionTests adds the +1).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/research/2026-05-21-a6-cdb-capture-findings.md

@@ -282,3 +282,75 @@ Per spec §5.1: "highest-confidence single-cause fix first."
 - **A6.P1.5** (optional, ~1 hour): extend cdb probe with paired entry/exit BPs to capture `adjust_sphere_to_plane` output delta (Table 1) and `find_collisions` exit-path discrimination (Table 2). Only needed if A6.P3 fixes don't close the symptoms and we need sharper data. Defer until after A6.P3 first attempt.
 - **Issue #95** (separate work surface): portal-graph visibility blowup. Schedule outside A6 since fixing it unblocks scen6-9 captures and any future dungeon physics work.
 - **Symptom roster update:** add "indoor sling-out" to M1.5 symptom list (Finding 3 family); already captured here as a finding, but M1.5 doc should reflect it.
+
+---
+
+## A6.P3 slice 1 — SHIPPED 2026-05-21
+
+Strip-synthesis + Mechanism B (LKCP restore) fix landed in 8 commits across this same session:
+
+| Commit | Task | What |
+|---|---|---|
+| `ba9655f` | plan | A6.P3 slice 1 implementation plan written |
+| `6b4be7f` + `c6bc2b9` | T1 | Research note: retail's `CTransition::validate_transition` LKCP-restore (line 272565-272583) + insertion-point identified in our `Transition.ValidateTransition` at TransitionTypes.cs:2849 |
+| `869edd9` | T2 | Test instrumentation: `CollisionInfo.ContactPlaneWriteCount` counter |
+| `36975ef` + `a32f569` | T3 | Failing regression: `IndoorContactPlaneRetentionTests` — asserts ≤5 CP writes across 60 flat-floor frames |
+| `5aba071` | T4 | Mechanism B (LKCP restore) inserted in `ValidateTransition` + proximity-check sphere bug fix (`GlobalSphere[0]` → `GlobalCurrCenter[0]`) |
+| `5f7722a` + `39fc037` + `bd5fe2e` | T5 | Indoor branch of `FindEnvCollisions` stripped to match retail's tiny `CEnvCell::find_env_collisions` shape; test redesigned as real regression sentinel (validated 60-writes-pre-strip → 0-writes-post-strip) |
+| `066568a` | T6/T7 partial | scen2_inn_stairs_postfix acdream capture proves stairs now work |
+| (this commit) | T6 + T8 | scen3_inn_2nd_floor_postfix capture + bookkeeping (findings doc + roadmap + CLAUDE.md + issues #96/#97 filed) |
+
+### scen3 re-capture results (postfix)
+
+scen3 (Holtburg inn 2nd floor flat-walk) re-captured in this slice-1 ship commit:
+
+| Metric | Pre-fix (4b5aebc) | Post-fix | Reduction |
+|---|---:|---:|---:|
+| acdream cp-write (absolute) | 86,748 | 25,082 | 3.5× |
+| acdream cell-cache events (proxy for session length) | 527 | 9,629 | 18× longer session |
+| **cp-write per cell-cache (normalized)** | **164.61** | **2.60** | **63.2× per-unit-of-activity** |
+| retail BP7 set_contact_plane | 0 | 0 | unchanged (oracle) |
+
+Per-unit-of-activity drop is the meaningful number — a longer post-fix session naturally accumulates more total writes, but the rate per "unit of activity" (cell-cache events ~ landblocks traversed) collapsed 63×.
+
+### scen2 re-capture results (postfix — UNEXPECTED WIN)
+
+scen2 (Holtburg inn stairs) acdream re-captured at commit `066568a`. **Pre-fix: physics hammered BSP trying to resolve stairs (failure mode). Post-fix: user walked up and down stairs multiple times with no failure.** Tag shape shifted:
+
+| Tag | Pre-fix (stair FAIL) | Post-fix (stair SUCCESS) | Signal |
+|---|---:|---:|---|
+| indoor-walkable | 859 | **0** | synthesis gone (as designed) |
+| push-back-cell | 1,478 | 879 (-40%) | multi-cell iteration relaxed |
+| push-back | 51 | 345 (+577%) | real step_up firing |
+| push-back-disp | 4,156 | 6,055 (+46%) | real BSP traversal |
+| cp-write | 33,969 | 57,846 | L622 seed (slice 2 work) |
+
+Stairs working post-slice-1 confirms A6.P2's hypothesis that **Finding 1 (dispatcher entry frequency mismatch) was a secondary effect of Finding 2** — fixing CP retention also closes the cell-array iteration storm that prevented stair-step resolution.
+
+### Visual verification (user happy-testing, 2026-05-21)
+
+User report from happy-testing session post-slice-1:
+- ✅ 2nd floor walking works (with caveats below)
+- ✅ Stairs up + down work (M1.5 demo target unblocked)
+- ✅ Cellar descent works (M1.5 demo target unblocked)
+- ❌ Phantom collisions occasionally on 2nd floor — filed as **issue #97** (hypothesis: caused by #96)
+- ❌ Occasional fall-through on 2nd floor — filed as **issue #97** (same)
+- ❌ See-through-walls indoors — **issue #95** (not A6 scope; visibility blowup)
+- ❌ Indoor lighting broken — **A7 scope**
+
+### Status of A6.P2 findings post-slice-1
+
+| Finding | Status post-slice-1 |
+|---|---|
+| Finding 1 — dispatcher entry frequency mismatch | **CLOSED as side-effect of Finding 2 fix** (scen2 dispatcher shape now retail-like) |
+| Finding 2 — ContactPlane resynthesis blowup | **PARTIALLY CLOSED.** Synthesis path eliminated (indoor-walkable = 0). Remaining 99.3% of post-fix CP writes come from `PhysicsEngine.ResolveWithTransition` line 622 — a per-tick body-CP seed that retail doesn't do. **Filed as issue #96** for slice 2. |
+| Finding 3 — Indoor cell-resolver sling-out | OPEN. Not addressed by slice 1. Needs scen4 re-capture to confirm whether sling-out symptom persists post-slice-1 (possible side-effect close); separate fix surface in ResolveCellId / CheckBuildingTransit otherwise. |
+| Finding 4 — Portal-graph visibility blowup | OPEN as issue #95 (not A6 scope; user-confirmed during happy-testing). |
+
+### Slice 2 recommendation
+
+**Highest-value next slice: gate the L622 per-tick CP seed.** It's responsible for 99.3% of remaining post-fix CP writes (24,906 of 25,082 in scen3 postfix). Retail's equivalent code path fires zero `set_contact_plane` calls during flat-floor walks. Either remove the seed entirely (rely on Mechanism A/B for CP propagation) OR gate it to fire only when the body's CP has changed since last seed.
+
+After slice 2, re-test phantom collisions + fall-through (issue #97) — they may close as side-effects (same family of "CP state being unstable across ticks"). If not, that becomes slice 3 territory + Finding 3 work.
+
+A6.P4 (workaround removal + visual verification) can proceed in parallel with slice 2 if scope allows.