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>

src/AcDream.App/Input/PlayerMovementController.cs

@@ -288,12 +288,29 @@ public sealed class PlayerMovementController
         _motion.apply_current_movement(cancelMoveTo: false, allowJump: false);
     }
 
+    // L.2a slice 1 (2026-05-12): centralized CellId mutation so the
+    // [cell-transit] probe fires from a single chokepoint. Both the
+    // server-snap path (SetPosition) and the per-frame resolver path
+    // route through here. When PhysicsDiagnostics.ProbeCellEnabled is
+    // off this collapses to a single bool-compare + assignment — zero
+    // logging cost.
+    private void UpdateCellId(uint newCellId, string reason)
+    {
+        if (newCellId != CellId && PhysicsDiagnostics.ProbeCellEnabled)
+        {
+            var pos = _body.Position;
+            Console.WriteLine(System.FormattableString.Invariant(
+                $"[cell-transit] 0x{CellId:X8} -> 0x{newCellId:X8} pos=({pos.X:F3},{pos.Y:F3},{pos.Z:F3}) reason={reason}"));
+        }
+        CellId = newCellId;
+    }
+
     public void SetPosition(Vector3 pos, uint cellId)
     {
         _body.Position = pos;
         _prevPhysicsPos = pos;
         _currPhysicsPos = pos;
-        CellId = cellId;
+        UpdateCellId(cellId, "teleport");
 
         // Treat as grounded after a server-side position snap.
         _body.TransientState = TransientStateFlags.Contact | TransientStateFlags.OnWalkable;
@@ -760,7 +777,7 @@ public sealed class PlayerMovementController
 
 
         _wasAirborneLastFrame = !_body.OnWalkable;
-        CellId = resolveResult.CellId;
+        UpdateCellId(resolveResult.CellId, "resolver");
 
         // ── 6. Determine outbound motion commands ─────────────────────────────
         uint? outForwardCmd = null;