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.Core/Physics/PhysicsDiagnostics.cs

@@ -0,0 +1,40 @@
+using System;
+
+namespace AcDream.Core.Physics;
+
+/// <summary>
+/// L.2a slice 1 (2026-05-12) — runtime-toggleable physics probe flags.
+/// Initialized from env vars at process start; flippable at runtime via
+/// the DebugPanel mirror (or by direct assignment). Log call sites read
+/// these statics so a checkbox toggle takes effect on the next resolve
+/// without relaunching.
+///
+/// <para>
+/// Slice 1 ships <see cref="ProbeResolveEnabled"/> +
+/// <see cref="ProbeCellEnabled"/>. Future slices may fold the older
+/// <c>ACDREAM_DUMP_*</c> env vars into this class for unified runtime
+/// toggling. Until then, those older flags remain sticky-at-startup
+/// per their original implementation.
+/// </para>
+/// </summary>
+public static class PhysicsDiagnostics
+{
+    /// <summary>
+    /// When true, <see cref="PhysicsEngine.ResolveWithTransition"/> emits
+    /// one structured <c>[resolve]</c> line per call: input + target +
+    /// output position/cell, grounded state, contact-plane status,
+    /// collision-normal validity, walkable polygon status, moving entity
+    /// id. Initial state from <c>ACDREAM_PROBE_RESOLVE=1</c>.
+    /// </summary>
+    public static bool ProbeResolveEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_RESOLVE") == "1";
+
+    /// <summary>
+    /// When true, every change to <c>PlayerMovementController.CellId</c>
+    /// emits one <c>[cell-transit]</c> line: old → new cell, current
+    /// world position, reason tag (<c>resolver</c> / <c>teleport</c>).
+    /// Initial state from <c>ACDREAM_PROBE_CELL=1</c>.
+    /// </summary>
+    public static bool ProbeCellEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_CELL") == "1";
+}

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -677,6 +677,28 @@ public sealed class PhysicsEngine
                 $"deltaXY=({dx:F3},{dy:F3}) cp={cpInfo}");
         }
 
+        // L.2a slice 1 (2026-05-12): general-purpose resolver probe.
+        // One line per call when PhysicsDiagnostics.ProbeResolveEnabled
+        // is set (env var ACDREAM_PROBE_RESOLVE=1 at startup, or the
+        // DebugPanel checkbox flipped at runtime). Captures every
+        // dimension L.2 cares about: input/output position, input/output
+        // cell, ok-vs-partial, grounded-in vs contact-out, contact-plane
+        // status, wall normal if hit, walkable polygon valid. Zero cost
+        // when off (one static-bool read).
+        if (PhysicsDiagnostics.ProbeResolveEnabled)
+        {
+            var probePost = sp.CheckPos;
+            string probeCp = ci.ContactPlaneValid
+                ? "valid"
+                : (ci.LastKnownContactPlaneValid ? "lastKnown" : "none");
+            string probeHit = collisionNormalValid
+                ? System.FormattableString.Invariant(
+                    $"yes n=({collisionNormal.X:F2},{collisionNormal.Y:F2},{collisionNormal.Z:F2})")
+                : "no";
+            Console.WriteLine(System.FormattableString.Invariant(
+                $"[resolve] ent=0x{movingEntityId:X8} in=({currentPos.X:F3},{currentPos.Y:F3},{currentPos.Z:F3}) cell=0x{cellId:X8} tgt=({targetPos.X:F3},{targetPos.Y:F3},{targetPos.Z:F3}) out=({probePost.X:F3},{probePost.Y:F3},{probePost.Z:F3}) cell=0x{sp.CheckCellId:X8} ok={ok} groundedIn={isOnGround} cp={probeCp} hit={probeHit} walkable={sp.HasLastWalkablePolygon}"));
+        }
+
         if (ok)
         {
             bool onGround = ci.ContactPlaneValid