feat(core): Phase B.3 — CellPortal-based indoor/outdoor transitions in PhysicsEngine

Replace the disabled if(false) outdoor→indoor branch with real portal-plane
crossing logic. LandblockPhysics now carries IReadOnlyList<PortalPlane> Portals
(populated at load time; GameWindow passes Array.Empty for now until Task 3).

Resolve logic:
- Outdoor player: tests all portals where TargetCellId==0xFFFF (outside-facing);
  crossing enters the portal's OwnerCellId.
- Indoor player: tests portals where OwnerCellId==currentCell; crossing to
  TargetCellId==0xFFFF exits to terrain, otherwise transitions room-to-room.
- Landblock boundary crossing: unchanged — candidatePos landblock lookup already
  picks the adjacent block's terrain naturally.

Tests: renamed disabled test → Resolve_OutdoorThroughPortal_TransitionsToIndoor;
added Resolve_IndoorThroughExitPortal_TransitionsToOutdoor and
Resolve_LandblockBoundary_PicksAdjacentTerrain. 274 tests green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/AcDream.Core.Tests/Input/PlayerMovementControllerTests.cs

@@ -17,7 +17,7 @@ public class PlayerMovementControllerTests
         for (int i = 0; i < 256; i++) heightTable[i] = i * 1f;
         var terrain = new TerrainSurface(heights, heightTable);
         engine.AddLandblock(0xA9B4FFFFu, terrain, Array.Empty<CellSurface>(),
-            worldOffsetX: 0f, worldOffsetY: 0f);
+            Array.Empty<PortalPlane>(), worldOffsetX: 0f, worldOffsetY: 0f);
         return engine;
     }
 
@@ -100,4 +100,82 @@ public class PlayerMovementControllerTests
 
         Assert.True(result.MotionStateChanged);
     }
+
+    [Fact]
+    public void Update_JumpOnFlatTerrain_BecomesAirborne()
+    {
+        var engine = MakeFlatEngine();
+        var controller = new PlayerMovementController(engine);
+        controller.SetPosition(new Vector3(96f, 96f, 50f), 0x0001);
+
+        var input = new MovementInput(Jump: true);
+        controller.Update(0.016f, input);
+
+        Assert.True(controller.IsAirborne);
+        Assert.True(controller.VerticalVelocity > 0f);
+    }
+
+    [Fact]
+    public void Update_AirborneFrames_ZRiseThenFalls()
+    {
+        var engine = MakeFlatEngine();
+        var controller = new PlayerMovementController(engine);
+        controller.SetPosition(new Vector3(96f, 96f, 50f), 0x0001);
+
+        // Jump
+        controller.Update(0.016f, new MovementInput(Jump: true));
+        float z1 = controller.Position.Z;
+
+        // A few frames of rising
+        controller.Update(0.1f, new MovementInput());
+        float z2 = controller.Position.Z;
+        Assert.True(z2 > z1, "Should be rising");
+
+        // Many frames — should come back down
+        for (int i = 0; i < 30; i++)
+            controller.Update(0.05f, new MovementInput());
+
+        Assert.False(controller.IsAirborne, "Should have landed");
+        Assert.Equal(50f, controller.Position.Z, precision: 1);
+    }
+
+    [Fact]
+    public void Update_WalkOffLedge_BecomesFalling()
+    {
+        // Build terrain with a sharp cliff: grid x<5 = Z50, grid x>=5 = Z20.
+        // heights[x*9+y] is indexed x-major; heightTable[i]=i*1f so
+        // byte value == Z value directly.
+        var heights = new byte[81];
+        for (int x = 0; x < 9; x++)
+            for (int y = 0; y < 9; y++)
+                heights[x * 9 + y] = (byte)(x < 5 ? 50 : 20);
+
+        var heightTable = new float[256];
+        for (int i = 0; i < 256; i++) heightTable[i] = i * 1f;
+
+        var engine = new PhysicsEngine();
+        var terrain = new TerrainSurface(heights, heightTable);
+        engine.AddLandblock(0xA9B4FFFFu, terrain, Array.Empty<CellSurface>(),
+            Array.Empty<PortalPlane>(), worldOffsetX: 0f, worldOffsetY: 0f);
+
+        // Position the player just before the cliff edge (localX=118 ≈ grid x=4.92).
+        // At this point terrain Z is ~51.7 (bilinear interpolation near the high side).
+        // One step at walk speed will cross into the low region where terrain drops
+        // ~28 units — more than StepUpHeight=5, triggering the ledge-fall.
+        var controller = new PlayerMovementController(engine);
+        controller.SetPosition(new Vector3(118f, 96f, 50f), 0x0001);
+        controller.Yaw = 0f; // facing +X
+
+        // Single step — should trigger airborne state because terrain drops sharply.
+        controller.Update(0.05f, new MovementInput(Forward: true));
+
+        Assert.True(controller.IsAirborne, "Player should be airborne after stepping off the cliff");
+
+        // Simulate enough frames to fall and land on the Z=20 floor.
+        for (int i = 0; i < 60; i++)
+            controller.Update(0.05f, new MovementInput(Forward: true));
+
+        Assert.False(controller.IsAirborne, "Player should have landed");
+        Assert.Equal(20f, controller.Position.Z, precision: 1);
+    }
 }