docs(phys L.2a): handoff + next-session prompt + CLAUDE.md / plan-of-record updates

Documents the L.2a-slice-1/2/3 ship for the next session and updates
the project's source-of-truth docs to reflect current state.

New files:
- docs/research/2026-05-12-l2a-shipped-l2d-handoff.md — full cold-start
  handoff: what shipped, three findings (L.2e cell-id format gap, L.2c
  wall-slide working, L.2d sub-direction = CBuildingObj port), branch
  state, diagnostic surface inventory, files changed, open concerns,
  cold-start checklist, reproduction recipe.
- docs/research/2026-05-12-l2d-next-session-prompt.md — terse copy-paste
  prompt for the next Claude Code session.

Modified:
- docs/plans/2026-04-29-movement-collision-conformance.md — added "Current
  sub-direction" note under L.2d capturing the evidence-driven call:
  building-mesh fidelity issue, not door-state-toggle.
- CLAUDE.md — updated "Currently between phases" → "Currently in Phase
  L.2"; added L.2a shipped paragraph; added ACDREAM_PROBE_RESOLVE /
  ACDREAM_PROBE_CELL to Diagnostic env vars; prepended L.2d brainstorm
  to next-phase candidates list.

No code changes in this commit.

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

CLAUDE.md

@@ -526,8 +526,31 @@ acdream's plan lives in two files committed to the repo:
   acceptance criteria. Do not drift from the spec without explicit user
   approval.
 
-**Currently between phases.** Phase C.1.5b just shipped; next phase is
-the user's call from the candidate list below.
+**Currently in Phase L.2 (Movement & Collision Conformance).** L.2a slices
+1+2+3 shipped 2026-05-12 (this evening); the natural next step is the
+L.2d slice 1 brainstorm / design spec. Cold-start prompt for the next
+session: [`docs/research/2026-05-12-l2d-next-session-prompt.md`](docs/research/2026-05-12-l2d-next-session-prompt.md).
+Full handoff: [`docs/research/2026-05-12-l2a-shipped-l2d-handoff.md`](docs/research/2026-05-12-l2a-shipped-l2d-handoff.md).
+
+**Phase L.2a (Truth & Diagnostics) slices 1-3 shipped 2026-05-12.**
+Three commits land the L.2 "make every bad movement outcome explainable"
+diagnostic foundation. Slice 1 (`ebef820`) adds runtime-toggleable
+`ACDREAM_PROBE_RESOLVE` (one `[resolve]` line per
+`PhysicsEngine.ResolveWithTransition` call) + `ACDREAM_PROBE_CELL` (one
+`[cell-transit]` line per `PlayerMovementController.CellId` change),
+both backed by a new `AcDream.Core.Physics.PhysicsDiagnostics` static
+class and mirrored as DebugPanel checkboxes. Slice 2 (`e0c08bc`) extends
+the `[resolve]` line with `obj=0x...` attribution. Slice 3 (`a068292`)
+populates the previously-stub `CollisionInfo.CollideObjectGuids` /
+`LastCollidedObjectGuid` (declared in `TransitionTypes.cs` but never
+written anywhere) at the per-object iteration in `FindObjCollisions`,
+so the slice-2 promise is now actually delivered. Visual-verified at
+the Holtburg Town doorway: probes captured 140 wall hits attributed to
+`obj=0xA9B47900` (landblock-baked static = the building itself,
+**NOT** a door entity), confirming L.2d sub-direction as **port
+`CBuildingObj` collision + per-cell walkability** rather than door-
+state-toggle. Plus a definitive L.2e finding: player `CellId` tracked
+as bare low byte (`0x00000029`) with no landblock prefix.
 
 **Phase C.1.5b (per-part PES transforms + dat-hydrated entity DefaultScript)
 shipped 2026-05-12.** Closes issue #56. `SetupPartTransforms.Compute(setup)`
@@ -585,6 +608,13 @@ together comprise the streaming + rendering perf foundation for the
 project.
 
 **Next phase candidates (in rough preference order):**
+- **L.2d slice 1 brainstorm + spec** (`docs/research/2026-05-12-l2d-next-session-prompt.md`).
+  Direct continuation of tonight's L.2a evidence: port `CBuildingObj` collision
+  + per-cell walkability so doorway gaps are walkable. Unblocks "walk into a
+  building" + sets up G.3 dungeon streaming. **Note:** triage the 8 pre-existing
+  test failures first (none introduced by L.2a slices — verified by stash + rerun
+  — but most touch movement/physics code L.2d will evolve). See the handoff doc's
+  "Open concerns" section.
 - **Triage the chronic open-issue list** in `docs/ISSUES.md` — #2 (lightning),
   #4 (sky horizon-glow), #28 (aurora), #29 (cloud thinness), #37 (humanoid
   coat), #50 (stray tree), #41 (remote-motion blips) have been open since
@@ -744,6 +774,18 @@ via `PlayerMovementController.ApplyServerRunRate`) or from
   diagnostics (`[UM_RAW]`, `[SCFAST]`, `[SCFULL]`, `[SETCYCLE]`,
   `[FWD_WIRE]`, `[OMEGA_DIAG]`, `[SEQSTATE]`, `[PARTSDIAG]`,
   `[VEL_DIAG]`, `[UPCYCLE]`). Heavy.
+- `ACDREAM_PROBE_RESOLVE=1` — L.2a slice 1+2+3 (2026-05-12). One
+  `[resolve]` line per `PhysicsEngine.ResolveWithTransition` call:
+  input + target + output position/cell, ok-vs-partial, grounded-in,
+  contact-plane status, wall normal if hit, **responsible entity
+  guid** (post-slice-3 attribution plumbing), env flag, walkable
+  polygon valid. Heavy (~30 Hz × every entity). Runtime-toggleable
+  via the DebugPanel "Diagnostics" section if `ACDREAM_DEVTOOLS=1`.
+- `ACDREAM_PROBE_CELL=1` — L.2a slice 1 (2026-05-12). One
+  `[cell-transit]` line per `PlayerMovementController.CellId`
+  change: old → new cell, world position, reason tag
+  (`resolver` / `teleport`). Low volume — only fires on actual cell
+  crossings. Runtime-toggleable via the same DebugPanel section.
 - *(retired 2026-05-05 by L.3 M2/M3)* `ACDREAM_INTERP_MANAGER` was an
   env-var gate on an experimental per-tick remote motion path. L.3 M2
   (commit 40d88b9) replaced both gates (`OnLivePositionUpdated` +