feat(phys L.2a slice 1): resolver + cell-transit probes (PhysicsDiagnostics)

New static `AcDream.Core.Physics.PhysicsDiagnostics` holds two
runtime-toggleable flags initialized from env vars:

- ACDREAM_PROBE_RESOLVE=1 — emit one [resolve] line per
  PhysicsEngine.ResolveWithTransition call: input/target/output
  position+cell, ok-vs-partial, grounded-in, contact-plane status,
  wall normal if hit, walkable-polygon valid, moving entity id.
- ACDREAM_PROBE_CELL=1 — emit one [cell-transit] line per
  PlayerMovementController.CellId change: old → new cell, current
  world position, reason tag (resolver / teleport).

Both also exposed as runtime-toggleable checkboxes in the DebugPanel
"Diagnostics" section. Unlike the existing four Dump-* checkboxes
(which only mirror sticky-at-startup env vars), the two new ones
forward directly to PhysicsDiagnostics — toggling on/off takes
effect on the next physics resolve, no relaunch.

Why now: L.2's plan-of-record (docs/plans/2026-04-29-movement-collision-
conformance.md) explicitly says "Land L.2a diagnostics first. Do not
make another physics change blind." This slice closes the most-load-
bearing gap in L.2a — a general-purpose probe on the resolver outcome
and a cell-transit log — so that later L.2b/c/d/e physics changes can
be evidence-driven instead of guessed. Foundation for the indoor /
dungeon walking trajectory (G.3 unblock).

Pure additive: when both flags are off (default), the probes collapse
to a single static-bool read per resolve, zero log cost. PlayerMovement
Controller's two CellId-mutation sites are now routed through a
private UpdateCellId(reason) helper for diag chokepoint.

Build green, 1032/1040 unit tests pass. The 8 failing tests are
pre-existing on the branch base (verified by stash-and-rerun);
none touch resolver or cell-transit code; all fail identically with
this slice stashed. Investigation deferred to a follow-up.

Refs: docs/plans/2026-04-29-movement-collision-conformance.md (L.2a
shipped-slice note added in same commit).

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

docs/plans/2026-04-29-movement-collision-conformance.md

@@ -92,6 +92,35 @@ Goal: make every bad movement outcome explainable.
 - Build real-DAT fixture capture for known walls, building ledges, rooftops,
   slopes, landblock seams, and dungeon entrances.
 
+Current shipped slices:
+
+- 2026-04-30: cdb + TTD retail-observer toolchain (`tools/pdb-extract/`,
+  `tools/ttd-record.ps1`, `tools/ttd-query.ps1`) with PDB pairing checker
+  and ring-buffer trace replay. The "retail observer harness" line item.
+- 2026-04 (pre-L.2 rename): `ACDREAM_DUMP_MOVE_TRUTH` paired
+  outbound/server-echo dumper in `GameWindow` covers outbound packet
+  fields + server echo + correction delta with cell-id mismatch.
+- Pre-L.2: scenario-specific dumps `ACDREAM_DUMP_MOTION`,
+  `ACDREAM_DUMP_STEEP_ROOF`, `ACDREAM_DUMP_STEPUP`,
+  `ACDREAM_DUMP_EDGE_SLIDE` for the codepaths hit during prior bug chases.
+- 2026-05-12 (slice 1): general-purpose probes via new
+  `AcDream.Core.Physics.PhysicsDiagnostics` static class.
+  `ACDREAM_PROBE_RESOLVE` emits one `[resolve]` line per
+  `PhysicsEngine.ResolveWithTransition` call (input/output pos+cell,
+  ok-vs-partial, grounded-in, contact-plane status, wall normal if hit,
+  walkable polygon valid, moving entity id).
+  `ACDREAM_PROBE_CELL` emits one `[cell-transit]` line per
+  `PlayerMovementController.CellId` change with old→new + position +
+  reason tag (`resolver`/`teleport`). Both flippable live via the
+  DebugPanel "Diagnostics" section — checkbox toggles take effect on
+  the next resolve, no relaunch required.
+
+Remaining L.2a work: contact-plane probe (general, not just steep-roof),
+ShadowObjectRegistry hit log ("you collided with entity X"), water probe,
+real-DAT fixture-capture pipeline, and folding the older sticky-at-startup
+`ACDREAM_DUMP_*` flags into `PhysicsDiagnostics` for unified runtime
+toggling.
+
 ### L.2b - Movement Wire / Contact Authority
 
 Goal: stop sending movement packets that claim more certainty than the local