fix #137 (corridor phantom resolved): slide_sphere opposing branch returns Collided; the 'wall' was synthetic

The mechanism-1 theory (PortalSide portal polys solid in our physics set)
is REFUTED for the corridor repro, and the remaining half of the phantom
is fixed — no cdb session needed:

- The live hit normal (-1.00,0.03,-0.03) matches NO dat polygon: a
  world-space sweep of both seam cells + every portal-adjacent neighbor
  (CorridorSeam_FindPolygonMatchingLiveHit) returns zero candidates. The
  normal is the negated movement direction — the SYNTHETIC value
  slide_sphere's opposing-normals branch records (reversed = -gDelta).
- Cell 0x8A02016E has IDENTITY rotation (the prior session's 'rotation
  maps the portal planes into the -X wall' was a misattribution). The
  PortalSide polys to 0x011E are +-Y planes 1.4 m beside the player's
  track, perpendicular to the +X run — pos_hits_sphere's directional
  cull rejects them for that movement. They ARE referenced by the dat's
  physics-BSP leaves (CorridorCell_PhysicsBspLeafMembership), so retail
  tests them too when approached into their plane; the dat's
  keep-PortalSide / strip-ExactMatch asymmetry reads as intentional
  (solid window/grate-class portals). No portal-poly filter — exactly
  the blanket-skip the pickup warned against.
- Port fix: CSphere::slide_sphere's opposing-normals branch
  (0x005375d7-0x0053762c) records the reversed displacement and returns
  COLLIDED_TS; our port returned OK ('retail returns OK here' was a
  decomp misread), letting the step complete as-is with the synthetic
  collision normal that validate's epilogue then persisted as the
  sliding normal the wedge absorbed on. TransitionTypes opposing branch
  now returns Collided; pinned by
  SlideSphere_OpposingNormals_ReturnsCollided_WithReversedDisplacementNormal
  (RED->GREEN).
- Dat-backed replay (Issue137CorridorSeamReplayTests) reproduces the
  live hit frame verbatim (same in/out to the millimeter, same 016E->017A
  transit, same +8mm settle) and runs the corridor CLEAN: hit=no, no
  sliding normal persisted, six further forward frames advance freely.
- Inspection tests extended: physics-BSP leaf membership walk +
  hit-normal candidate sweep + downward-poly sweep (all report-style,
  dat-gated). Pickup prompt banner'd SUPERSEDED; ISSUES #137 updated
  (door half stays open); audit doc extended with the resolution.

Suites: Core 2551 / App 713 / UI 425 / Net 385, 0 failures.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/AcDream.Core.Tests/Physics/Issue137SlidingNormalLifecycleTests.cs

@@ -172,6 +172,42 @@ public class Issue137SlidingNormalLifecycleTests
         Assert.Equal(TransitionState.Collided, result);
     }
 
+    /// <summary>
+    /// <c>CSphere::slide_sphere</c>'s opposing-normals branch (collision
+    /// normal anti-parallel to the contact plane — e.g. a ceiling-facing
+    /// normal while grounded) records the REVERSED displacement as the
+    /// collision normal and returns <b>COLLIDED_TS</b> — retail 0x00537440
+    /// @0x005375d7-0x0053762c: <c>*normal = −gDelta; normalize;
+    /// set_collision_normal; return 2</c>. Our port returned OK (its comment
+    /// even claimed "retail returns OK here"), letting the step complete
+    /// as-is with a synthetic reversed-movement collision normal — the exact
+    /// signature of the live corridor hit (`hit=yes n=(−1.00,0.03,−0.03)` =
+    /// the negated run direction, matching NO dat polygon).
+    /// </summary>
+    [Fact]
+    public void SlideSphere_OpposingNormals_ReturnsCollided_WithReversedDisplacementNormal()
+    {
+        var t = new Transition();
+        t.SpherePath.InitPath(
+            new Vector3(0f, 0f, 0.2f), new Vector3(0.3f, 0f, 0.2f),
+            0xA9B40001u, BSPStepUpFixtures.SphereRadius);
+        t.CollisionInfo.SetContactPlane(new Plane(Vector3.UnitZ, 0f), 0xA9B40001u, false);
+
+        // Make gDelta exactly (0.4, 0, 0): currPos = check sphere − (0.4,0,0).
+        var currPos = t.SpherePath.GlobalSphere[0].Origin - new Vector3(0.4f, 0f, 0f);
+
+        // Downward collision normal vs the +Z contact plane → cross ≈ 0
+        // (parallel), dot = −1 < 0 (opposing) → the reverse branch.
+        var result = t.SlideSphereInternal(new Vector3(0f, 0f, -1f), currPos);
+
+        Assert.Equal(TransitionState.Collided, result);
+        Assert.True(t.CollisionInfo.CollisionNormalValid);
+        Assert.True(t.CollisionInfo.CollisionNormal.X < -0.99f,
+            $"Collision normal must be the normalized reversed displacement " +
+            $"(−1,0,0); got ({t.CollisionInfo.CollisionNormal.X:F3}," +
+            $"{t.CollisionInfo.CollisionNormal.Y:F3},{t.CollisionInfo.CollisionNormal.Z:F3}).");
+    }
+
     // =========================================================================
     // Engine-level lifecycle pin — the retail persist/absorb/clear cycle at a
     // REAL wall. Guards the fix against regressing wall behavior, and