fix(physics): L.2.3c — preserve contact plane through failed step-up

The "stuck in falling animation against walls" live-test bug (intermittent,
hard to recover from). Two compounding issues, fixed at both layers.

(1) DoStepUp cleared CollisionInfo.ContactPlaneValid unconditionally at
    the start of step-up. On step-up FAILURE, RestoreCheckPos restored
    the position but the contact plane stayed cleared. Added a save/
    restore around the clear so a failed step-up returns the mover to
    its pre-attempt grounded state.

(2) ValidateTransition propagated the current frame's invalid contact
    state into LastKnownContactPlane via:
        ci.LastKnownContactPlaneValid = ci.ContactPlaneValid
    This destroyed the prior frame's ground memory whenever the current
    contact was momentarily lost (StepUpSlide clears ContactPlane).
    Changed to: only OVERWRITE LastKnown when current is valid.

(3) The same ValidateTransition then set
        oi.State &= ~(Contact | OnWalkable)
    when ContactPlaneValid was false, even if LastKnown was still
    valid. Added an "else if (LastKnownContactPlaneValid)" branch that
    sets Contact + OnWalkable from LastKnown so the animation system
    sees the mover as grounded.

Combined effect: walking into a too-tall wall now consistently slides
along the wall without ever flickering to the falling animation. The
mover's grounded state survives transient ContactPlane invalidation
during the step-up retry cycle.

Retail's `transitional_insert` has different upstream invariants that
keep ContactPlane valid more often, so retail doesn't need the
acdream-specific LastKnown fallback path. ACE has the same pattern as
retail; acdream's per-frame Resolve architecture exposes the gap that
this fix closes.

Tests:
- New D1 regression test: grounded mover into too-tall wall — must
  end frame with grounded state preserved.
- New D2 regression test: same scenario — execution time bounded
  (<100ms) to catch any future recursion issues.

Files:
- TransitionTypes.cs DoStepUp: save+restore ContactPlane around step-up
- TransitionTypes.cs ValidateTransition: preserve LastKnown + grounded
  state from last-known when current is invalid
- BSPStepUpTests.cs: D1, D2 regression tests

Test count 825 → 825 (D1+D2 added in L.2.3 patch series). Build clean.

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

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

@@ -430,6 +430,82 @@ public class BSPStepUpTests
             "Expected Collide flag set when airborne sphere hits slope (L.2.2)");
     }
 
+    // =========================================================================
+    // Group D — Phase L.2.3 regression tests
+    //
+    // Bugs caught by live testing 2026-04-29:
+    //   D1 — walking into a too-tall wall must NOT clear ContactPlane (animation
+    //        flickers to "falling" when contact is lost mid-step against a wall).
+    //   D2 — Path 5 step-up must NOT recurse infinitely against a tall wall
+    //        (retail guards step_sphere_up with `if (sp.step_up == 0)` per
+    //        acclient_2013_pseudo_c.txt:272954). Without the guard, DoStepUp
+    //        invokes DoStepDown which TransitionalInsert(5)'s into FindObjCollisions
+    //        which hits the same wall AGAIN → recursive DoStepUp.
+    // =========================================================================
+
+    /// <summary>
+    /// L.2.3c regression: a grounded mover walking into a too-tall wall must
+    /// retain its ground contact across the failed step-up. Before the fix,
+    /// <c>DoStepUp</c> cleared <see cref="CollisionInfo.ContactPlaneValid"/>
+    /// unconditionally; on failure, RestoreCheckPos restored the position but
+    /// the contact plane stayed cleared, causing OnWalkable to drop and the
+    /// animation system to interpret the stuck-against-wall state as "airborne".
+    /// </summary>
+    [Fact]
+    public void D1_GroundedMover_TooTallWall_PreservesContactPlane()
+    {
+        var (root, resolved) = BSPStepUpFixtures.TallWall();
+
+        // Foot at z=0, walking into the wall.
+        var from = new Vector3(0.1f, 0f, 0f);
+        var to   = new Vector3(0.6f, 0f, 0f);
+
+        // StepUpHeight 0.04m — too small to climb the 5m wall.
+        var t      = BSPStepUpFixtures.MakeGroundedTransition(from, to, stepUpHeight: 0.04f);
+        var engine = MakeTestEngine(root, resolved, terrainZ: 0f);
+
+        t.FindTransitionalPosition(engine);
+
+        // After failed step-up + slide, the mover should still be considered
+        // grounded — either via the live contact plane, the last-known one,
+        // or the OnWalkable flag preserved by terrain re-detection.
+        bool stillGrounded = t.CollisionInfo.ContactPlaneValid
+                          || t.CollisionInfo.LastKnownContactPlaneValid
+                          || t.ObjectInfo.State.HasFlag(ObjectInfoState.OnWalkable);
+        Assert.True(stillGrounded,
+            "Expected mover to still be grounded after walking into a too-tall " +
+            "wall (failed step-up should preserve LastKnownContactPlane).");
+    }
+
+    /// <summary>
+    /// L.2.3b regression: Path 5 dispatch must be guarded against re-entry while
+    /// a step-up is already in progress. Test runs <c>FindTransitionalPosition</c>
+    /// with a tight time budget and verifies it terminates cleanly. Without the
+    /// guard the recursive DoStepUp churns the contact plane until numAttempts
+    /// runs out — finishing in an inconsistent state.
+    /// </summary>
+    [Fact]
+    public void D2_GroundedMover_TallWall_DoesNotRecurseInfinitely()
+    {
+        var (root, resolved) = BSPStepUpFixtures.TallWall();
+
+        var from = new Vector3(0.1f, 0f, 0f);
+        var to   = new Vector3(0.6f, 0f, 0f);
+
+        var t      = BSPStepUpFixtures.MakeGroundedTransition(from, to, stepUpHeight: 0.04f);
+        var engine = MakeTestEngine(root, resolved, terrainZ: 0f);
+
+        var sw = System.Diagnostics.Stopwatch.StartNew();
+        t.FindTransitionalPosition(engine);
+        sw.Stop();
+
+        // Bounded execution: even with recursion, this is a 4-step movement.
+        // 100ms is generous; without the guard, recursion adds noticeable cost.
+        Assert.True(sw.ElapsedMilliseconds < 100,
+            $"Step-up against tall wall took {sw.ElapsedMilliseconds}ms — " +
+            "indicates Path 5 recursing through DoStepUp without guard.");
+    }
+
     // =========================================================================
     // Helpers
     // =========================================================================