fix(test): A6.P3 slice 1 T5 — redesign test to actually catch synthesis regression

Code-review feedback on commit 5f7722a: the test was gamed —
placing the sphere exactly on the floor (worldPosZ = floorZ) made it
pass regardless of whether synthesis was present. With sphere center at
Z=0.48 (= floorZ + SphereRadius), PolygonHitsSpherePrecise's distance
guard fires immediately (|dist|=0.48 > rad=0.478) and
TryFindIndoorWalkablePlane returns false even WITH synthesis code. The
test would have passed even if the strip were reverted.

Redesign: restore worldPosZ = floorZ - 0.05f (sphere center at Z=0.43).
Now |dist|=0.43 < rad=0.478 → the guard passes → TryFindIndoorWalkablePlane
finds the floor polygon → synthesis would fire → CP writes every frame.
Path 5 (Contact branch) is not a concern: the loop moves only in X so
movement = (0.001, 0, 0), Dot(movement, floor_normal=(0,0,1)) = 0 ≥ 0 →
PosHitsSphere front-face cull rejects the floor hit even with sphere
center below the floor. Path 5 returns OK with zero CP writes. Contact
flag is left set to keep the test on the realistic grounded-mover path.

Validated locally by temporarily re-introducing the synthesis call —
test fails with 60 writes (1 per frame) pre-strip, passes with 0
additional writes post-strip. Now a real regression sentinel.

1148 pass + 8 pre-existing fail baseline maintained.

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

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

@@ -230,30 +230,38 @@ public class IndoorContactPlaneRetentionTests
     public void IndoorFlatFloorWalking_60Frames_ProducesAtMost5ExtraCpWrites()
     {
         // ── Arrange ───────────────────────────────────────────────────────────
-        // Post-fix grounded model:
+        // Geometry: sphere center 5 cm BELOW the floor so that the synthesis
+        // path's distance guard passes and TryFindIndoorWalkablePlane would
+        // actually find the floor polygon if synthesis were re-introduced.
         //
-        //   SpherePath.InitPath sets LocalSphere[0].Origin = (0,0,sphereRadius).
+        //   SpherePath.InitPath sets LocalSphere[0].Origin = (0,0,SphereRadius).
         //   After SetCheckPos(worldPos), the global sphere CENTER is at
-        //     worldPos + (0,0,SphereRadius) = (0,0,SphereRadius).
+        //     worldPos + (0,0,SphereRadius) = (worldPosX, 0, -0.05 + 0.48 = 0.43).
         //
-        //   A correctly-grounded sphere has its bottom exactly at the floor:
+        //   PolygonHitsSpherePrecise distance guard (BSPQuery.cs line ~117):
-        //     sphereBottom = sphereCenter.Z - SphereRadius
+        //     dist = Dot(normal=(0,0,1), center=(x,0,0.43)) + D(=0) = 0.43
-        //                  = worldPosZ + SphereRadius - SphereRadius
+        //     rad  = SphereRadius - EPSILON = 0.48 - 0.002 = 0.478
-        //                  = worldPosZ
+        //     |dist| = 0.43 < 0.478 → guard passes → polygon is tested → FOUND.
+        //   So TryFindIndoorWalkablePlane WOULD call ValidateWalkable → CP write
+        //   if the synthesis call were present. With the strip in place it is never
+        //   called → ≤5 additional writes → PASS.
         //
-        //   With worldPosZ = 0 (= floorZ), the bottom just touches the floor.
+        //   With the post-5f7722a setup (worldPosZ = floorZ = 0):
-        //   SphereIntersectsPolyInternal uses a strict penetration check, so a
+        //     dist = 0.48, rad = 0.478, |dist| = 0.48 > 0.478 → guard FIRES →
-        //   sphere touching-but-not-penetrating does NOT count as a hit.
+        //     TryFindIndoorWalkablePlane returns false even WITH synthesis code.
-        //   Path 5 (Contact grounded) returns OK with no CP write.
+        //   That setup was not a regression sentinel; this one is.
         //
-        //   Pre-fix: the sphere was positioned 5 cm BELOW the floor so that
+        //   Path 5 (BSP Contact branch, BSPQuery.cs ~line 1732):
-        //   TryFindIndoorWalkablePlane → ValidateWalkable would fire every frame.
+        //   The loop advances only in X, so movement = (0.001, 0, 0).
-        //   Post-fix: synthesis is gone; the sphere must be at its natural
+        //   PosHitsSphere culls hits where Dot(movement, normal) >= 0.
-        //   grounded position (bottom at floorZ) so that BSP Path 5 finds no
+        //   Dot((0.001,0,0), (0,0,1)) = 0 >= 0 → floor polygon is ALWAYS
-        //   penetration and returns OK immediately — zero additional CP writes.
+        //   rejected by the front-face cull, even though the sphere center is
-        const float floorZ       = 0f;
+        //   below the floor. Path 5 exits OK with no CP write regardless of
-        const float worldPosZ    = floorZ;               // sphere bottom exactly at floor
+        //   whether the Contact flag is set. Leaving Contact set keeps this
-        const float sphereCenterZ = worldPosZ + SphereRadius; // = 0.48
+        //   test on the realistic grounded-mover path.
+        const float floorZ        = 0f;
+        const float worldPosZ     = floorZ - 0.05f;          // 5 cm below floor
+        const float sphereCenterZ = worldPosZ + SphereRadius; // = 0.43
 
         var floorPlane = new Plane(Vector3.UnitZ, -floorZ);   // N·p + D = 0 → D = 0
         var worldPos   = new Vector3(0f, 0f, worldPosZ);
@@ -278,7 +286,7 @@ public class IndoorContactPlaneRetentionTests
         // indoor branch would call it each physics frame.
         for (int frame = 0; frame < SimulatedFrames; frame++)
         {
-            // Advance position by 1 mm forward — same Z (grounded on floor).
+            // Advance position by 1 mm forward — same Z (sphere center 5 cm below floor).
             var newPos = new Vector3(frame * 0.001f, 0f, worldPosZ);
             t.SpherePath.SetCheckPos(newPos, IndoorCellId);