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>

src/AcDream.Core/Physics/PhysicsDiagnostics.cs

@@ -10,11 +10,11 @@ namespace AcDream.Core.Physics;
 /// 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.
+/// L.2d slice 1 (2026-05-13) adds <see cref="ProbeBuildingEnabled"/> +
+/// the <see cref="LastBspHitPoly"/> diagnostic side-channel. 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
@@ -37,4 +37,61 @@ public static class PhysicsDiagnostics
     /// </summary>
     public static bool ProbeCellEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_CELL") == "1";
+
+    /// <summary>
+    /// L.2d slice 1 (2026-05-13). When true, every BSP-shadow-entry hit
+    /// attributed by <c>TransitionTypes.FindObjCollisions</c> emits a
+    /// multi-line <c>[resolve-bldg]</c> entry: which part (partIdx vs 0),
+    /// physics-BSP root radius vs visual AABB radius, world-space entity
+    /// origin, and the specific hit polygon's vertices in both
+    /// object-local and world space. Designed to distinguish the three
+    /// L.2d hypotheses (wrong BSP loaded / over-registered parts /
+    /// BSPQuery flaw) from a single Holtburg-doorway capture.
+    ///
+    /// <para>
+    /// Also gates a one-time <c>[entity-source]</c> log line at every
+    /// <c>ShadowObjects.Register(...)</c> call site in <c>GameWindow</c>
+    /// — makes <c>entityId=0xA9B479</c> in a probe line greppable to its
+    /// source registration within the same log file.
+    /// </para>
+    ///
+    /// <para>
+    /// Initial state from <c>ACDREAM_PROBE_BUILDING=1</c>. Mirrorable
+    /// via <c>DebugVM.ProbeBuilding</c> when <c>ACDREAM_DEVTOOLS=1</c>.
+    /// </para>
+    ///
+    /// <para>
+    /// Spec: <c>docs/superpowers/specs/2026-05-13-l2d-cbuildingobj-collision-design.md</c>.
+    /// </para>
+    /// </summary>
+    public static bool ProbeBuildingEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_BUILDING") == "1";
+
+    /// <summary>
+    /// L.2d slice 1 (2026-05-13). Diagnostic side-channel: the
+    /// <see cref="ResolvedPolygon"/> that <see cref="BSPQuery"/>
+    /// recorded for the most recent collision-normal write.
+    /// <see cref="TransitionTypes.FindObjCollisions"/> clears this to
+    /// <see langword="null"/> before each shadow-entry test and reads it
+    /// back after, so emitting the <c>[resolve-bldg]</c> probe line can
+    /// reference the actual hit poly without plumbing an out-param
+    /// through BSPQuery's recursive private methods.
+    ///
+    /// <para>
+    /// Written by <see cref="BSPQuery"/> only when
+    /// <see cref="ProbeBuildingEnabled"/> is true, so this stays
+    /// zero-cost in normal play. Cylinder collisions leave this
+    /// <see langword="null"/> — the probe line emits
+    /// <c>hitPoly: n/a (cylinder)</c> in that case.
+    /// </para>
+    ///
+    /// <para>
+    /// Not threadsafe — physics runs on a single thread. If that
+    /// changes, this needs <c>[ThreadStatic]</c> or rethink. Deviation
+    /// from spec component 4 (which described an out-param); the
+    /// side-channel keeps BSPQuery's signature stable and the diagnostic
+    /// path off the production code surface.
+    /// </para>
+    /// </summary>
+    public static ResolvedPolygon? LastBspHitPoly { get; set; }
 }