feat(phys): A6.P3 slice 1 step 1 — add Mechanism B (LKCP restore)

Restores CollisionInfo.ContactPlane from LastKnownContactPlane when:
  - LKCP is valid
  - the sphere's current center is geometrically close to the LKCP
    plane (|dot(global_curr_center, N) + d| <= radius + EPSILON)

Matches retail's validate_transition LKCP-restore at
acclient_2013_pseudo_c.txt:272577 (CTransition::validate_transition,
address 0050aa70, lines 272565-272582). Slice 1 step 1 of the
A6.P3 indoor CP retention fix. Step 2 (Task 5) strips the
TryFindIndoorWalkablePlane synthesis from FindEnvCollisions.

Also fixes the proximity-check sphere: was using
sp.GlobalSphere[0].Origin (start sphere); now uses
sp.GlobalCurrCenter[0].Origin (current center) per retail
(acclient_2013_pseudo_c.txt:272568).

Tests: 1147 pass, 9 fail (8 pre-existing + 1 IndoorContactPlaneRetention
from T3 — expected; T5 lands the actual synthesis-strip fix).

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

src/AcDream.Core/Physics/TransitionTypes.cs

@@ -2849,13 +2849,38 @@ public sealed class Transition
             //
             // Matches ACE PhysicsObj's pre-reuse check on the last-known
             // plane and retail's CPhysicsObj::get_object_info logic.
-            var sphereCenter = sp.GlobalSphere[0].Origin;
+            // A6.P3 slice 1 (2026-05-21). Retail uses global_curr_center (NOT
+            // global_sphere->center) for this proximity check — see
+            // acclient_2013_pseudo_c.txt:272568. global_sphere is the START
+            // sphere of the transition; global_curr_center is the CURRENT center
+            // after sub-step accumulation. Using the wrong one made the proximity
+            // guard fire on the wrong reference point.
+            var sphereCenter = sp.GlobalCurrCenter[0].Origin;
             var radius       = sp.GlobalSphere[0].Radius;
             float angle = Vector3.Dot(ci.LastKnownContactPlane.Normal, sphereCenter)
                           + ci.LastKnownContactPlane.D;
 
             if (radius + PhysicsGlobals.EPSILON > MathF.Abs(angle))
             {
+                // ── Mechanism B — restore CP from LKCP per retail ────────────────
+                // A6.P3 slice 1 (2026-05-21). Retail oracle:
+                // acclient_2013_pseudo_c.txt:272577 (inside CTransition::validate_transition
+                // at line 272547). When the sphere is geometrically close to the
+                // LastKnownContactPlane, retail restores CP from LKCP via
+                // set_contact_plane(&collision_info, &last_known_contact_plane,
+                // last_known_contact_plane_is_water). This closes the gap that the
+                // stripped TryFindIndoorWalkablePlane synthesis path used to fill —
+                // when no fresh Path-6 CP write lands in this transition, CP is
+                // retained from the previous frame instead of being re-synthesized.
+                //
+                // NOTE: SetContactPlane also re-latches LKCP fields
+                // (TransitionTypes.cs:258-261), which is a no-op here since we
+                // pass LKCP as the source.
+                ci.SetContactPlane(
+                    ci.LastKnownContactPlane,
+                    ci.LastKnownContactPlaneCellId,
+                    ci.LastKnownContactPlaneIsWater);
+
                 // Still close enough to the last-known plane — preserve
                 // grounded state. L.2.3i FloorZ test for OnWalkable.
                 oi.State |= ObjectInfoState.Contact;