feat(phys L.2d slice 1): BSP-hit diagnostic probe + plan-of-record correction

Adds ACDREAM_PROBE_BUILDING — a read-only per-shadow-entry probe that
captures full BSP collision evidence whenever TransitionTypes.FindObjCollisions
attributes a hit (via the existing L.2a slice 3 chain). One multi-line
[resolve-bldg] entry per attributed hit: partIdx, hasPhys, bspR vs
vAabbR, world-space entOrigin_lb, and the actual hit polygon's vertices
in both object-local and world space.

Paired with a one-time [entity-source] line at every ShadowObjects.Register
call site in GameWindow so entityId from a probe line is greppable to its
WorldEntity source within a single log file.

Plumbing: BSPQuery writes the resolved hit polygon to a new
PhysicsDiagnostics.LastBspHitPoly side-channel at the 5 SetCollisionNormal
sites in Paths 5/6 + CollideWithPt. TransitionTypes clears that field
before each shadow-entry dispatch and reads it back at the L.2a slice 3
attribution site to emit the probe line.

Spec component 4 originally described an out ResolvedPolygon? parameter
on BSPQuery.FindCollisions; the static side-channel achieves the same
observable behavior without plumbing through BSPQuery's recursive private
methods. Deviation noted in PhysicsDiagnostics.LastBspHitPoly's XML doc.

Reframes the plan-of-record's L.2d sub-direction paragraph: the 2026-05-12
handoff proposed porting CBuildingObj + per-cell walkability, but ACE
BuildingObj.cs:39-52 + named-retail acclient_2013_pseudo_c.txt:701260
show find_building_collisions is one BSP test on Parts[0]. Per-cell
walkability belongs to L.2e, not L.2d. L.2d slice 1 is the diagnostic;
slice 2 is the actual fix scoped from slice 1's evidence (one of three
hypotheses: wrong BSP loaded / over-registered parts / BSPQuery flaw).

Tests: 2 synthetic unit tests in PhysicsDiagnosticsTests.cs pin the
static API contract that the BSPQuery → side-channel → TransitionTypes
emission chain depends on. The multi-line line format itself is verified
by acceptance criterion 2 (live Holtburg-doorway capture) — covering it
here would require a heavy PhysicsEngine + Transition fixture for a
diagnostic-only emission.

Verified: dotnet build green; the 2 new tests pass; the 8 pre-existing
test failures listed in the L.2a handoff (MotionInterpreter GetMaxSpeed_*,
PositionManager.ComputeOffset_BothActive_Combined,
PlayerMovementController.Update_ForwardInput_*, Dispatcher.W_held_*,
BSPStepUpTests.{D4,C3}) remain failing — none introduced by this slice.

Spec: docs/superpowers/specs/2026-05-13-l2d-cbuildingobj-collision-design.md
Conformance anchors:
- acclient_2013_pseudo_c.txt:701260 (CBuildingObj::find_building_collisions)
- acclient_2013_pseudo_c.txt:323725 (BSPTREE::find_collisions)
- ACE references/ACE/Source/ACE.Server/Physics/Common/BuildingObj.cs:39-52

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

tests/AcDream.Core.Tests/Physics/PhysicsDiagnosticsTests.cs

@@ -0,0 +1,98 @@
+using AcDream.Core.Physics;
+using DatReaderWriter.Enums;
+using System.Numerics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+/// <summary>
+/// L.2d slice 1 (2026-05-13) — unit coverage for the new
+/// <see cref="PhysicsDiagnostics.ProbeBuildingEnabled"/> flag and
+/// <see cref="PhysicsDiagnostics.LastBspHitPoly"/> diagnostic
+/// side-channel.
+///
+/// <para>
+/// The full multi-line <c>[resolve-bldg]</c> format itself is verified
+/// by the slice's acceptance criterion #2 (live Holtburg-doorway
+/// capture) — covering it here would require a heavy
+/// <c>PhysicsEngine</c> + <c>ShadowObjectRegistry</c> + <c>Transition</c>
+/// fixture for what's a diagnostic-only emission. These tests pin the
+/// static API contract that the emission code depends on; if either of
+/// these tests breaks the emission will start producing stale data or
+/// failing to emit at all.
+/// </para>
+/// </summary>
+public class PhysicsDiagnosticsTests
+{
+    // -----------------------------------------------------------------------
+    // ProbeBuildingEnabled — flag gates the emission path.
+    // -----------------------------------------------------------------------
+
+    [Fact]
+    public void ProbeBuilding_StaticApi_Roundtrip()
+    {
+        bool initial = PhysicsDiagnostics.ProbeBuildingEnabled;
+        try
+        {
+            PhysicsDiagnostics.ProbeBuildingEnabled = true;
+            Assert.True(PhysicsDiagnostics.ProbeBuildingEnabled);
+
+            PhysicsDiagnostics.ProbeBuildingEnabled = false;
+            Assert.False(PhysicsDiagnostics.ProbeBuildingEnabled);
+        }
+        finally
+        {
+            // Restore so a process-wide static doesn't leak between tests
+            // (env-var init was the only thing that set this before).
+            PhysicsDiagnostics.ProbeBuildingEnabled = initial;
+        }
+    }
+
+    // -----------------------------------------------------------------------
+    // LastBspHitPoly — side-channel set by BSPQuery, read by FindObjCollisions.
+    //
+    // TransitionTypes.FindObjCollisions clears this to null before each
+    // shadow-entry dispatch; BSPQuery writes to it on hit when the probe is
+    // on; the emission site reads it. A failure here means the side-channel
+    // can't carry data through the call chain.
+    // -----------------------------------------------------------------------
+
+    [Fact]
+    public void LastBspHitPoly_StaticApi_Roundtrip()
+    {
+        ResolvedPolygon? initial = PhysicsDiagnostics.LastBspHitPoly;
+        try
+        {
+            PhysicsDiagnostics.LastBspHitPoly = null;
+            Assert.Null(PhysicsDiagnostics.LastBspHitPoly);
+
+            var synthetic = new ResolvedPolygon
+            {
+                Vertices = new[]
+                {
+                    new Vector3(-1f, 0f, 0f),
+                    new Vector3( 1f, 0f, 0f),
+                    new Vector3( 1f, 0f, 2f),
+                    new Vector3(-1f, 0f, 2f),
+                },
+                Plane     = new System.Numerics.Plane(0f, 1f, 0f, -94.123f),
+                NumPoints = 4,
+                SidesType = CullMode.None,
+            };
+            PhysicsDiagnostics.LastBspHitPoly = synthetic;
+
+            var read = PhysicsDiagnostics.LastBspHitPoly;
+            Assert.NotNull(read);
+            Assert.Equal(4, read!.NumPoints);
+            Assert.Equal(synthetic.Plane.D, read.Plane.D);
+            Assert.Same(synthetic, read);
+
+            PhysicsDiagnostics.LastBspHitPoly = null;
+            Assert.Null(PhysicsDiagnostics.LastBspHitPoly);
+        }
+        finally
+        {
+            PhysicsDiagnostics.LastBspHitPoly = initial;
+        }
+    }
+}