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>

src/AcDream.App/Input/PlayerMovementController.cs

@@ -15,7 +15,8 @@ public readonly record struct MovementInput(
     bool TurnLeft = false,
     bool TurnRight = false,
     bool Run = false,
-    float MouseDeltaX = 0f);
+    float MouseDeltaX = 0f,
+    bool Jump = false);
 
 /// <summary>
 /// Result of a single frame's movement update.
@@ -53,6 +54,12 @@ public sealed class PlayerMovementController
     /// </summary>
     public float StepUpHeight { get; set; } = 5.0f;
 
+    public float VerticalVelocity { get; private set; }
+    public bool IsAirborne { get; private set; }
+    public float JumpImpulse { get; set; } = 10f;
+    public float GravityAccel { get; set; } = 20f;
+    public float AirControlFactor { get; set; } = 0.2f;
+
     public float Yaw { get; set; }
     public Vector3 Position { get; private set; }
     public uint CellId { get; private set; }
@@ -104,11 +111,59 @@ public sealed class PlayerMovementController
         if (input.StrafeRight) { dx += rightX * speed * dt * 0.5f; dy += rightY * speed * dt * 0.5f; }
         if (input.StrafeLeft) { dx -= rightX * speed * dt * 0.5f; dy -= rightY * speed * dt * 0.5f; }
 
-        var delta = new Vector3(dx, dy, 0f);
+        // While airborne, reduce horizontal authority.
+        float airFactor = IsAirborne ? AirControlFactor : 1f;
+        var delta = new Vector3(dx * airFactor, dy * airFactor, 0f);
 
         // 3. Resolve via physics engine.
         var result = _physics.Resolve(Position, CellId, delta, StepUpHeight);
-        Position = result.Position;
+
+        // 4. Jump + gravity + falling (vertical axis).
+        float resolvedGroundZ = result.Position.Z;
+
+        // Initiate jump when grounded and Space pressed.
+        if (input.Jump && result.IsOnGround && !IsAirborne)
+        {
+            VerticalVelocity = JumpImpulse;
+            IsAirborne = true;
+        }
+
+        float newZ;
+        if (IsAirborne)
+        {
+            // Apply gravity and integrate vertical position.
+            VerticalVelocity -= GravityAccel * dt;
+            float candidateZ = Position.Z + VerticalVelocity * dt;
+
+            if (candidateZ <= resolvedGroundZ)
+            {
+                // Landed on terrain/floor.
+                newZ = resolvedGroundZ;
+                IsAirborne = false;
+                VerticalVelocity = 0f;
+            }
+            else
+            {
+                // Still airborne — override terrain Z with ballistic Z.
+                newZ = candidateZ;
+            }
+        }
+        else
+        {
+            // Detect walking off a ledge: terrain dropped more than StepUpHeight.
+            if (resolvedGroundZ < Position.Z - StepUpHeight)
+            {
+                IsAirborne = true;
+                VerticalVelocity = 0f;
+                newZ = Position.Z;  // stay at current Z this frame; gravity will pull us down next frame
+            }
+            else
+            {
+                newZ = resolvedGroundZ;
+            }
+        }
+
+        Position = new Vector3(result.Position.X, result.Position.Y, newZ);
         CellId = result.CellId;
 
         // 4. Determine current motion commands.
@@ -180,9 +235,9 @@ public sealed class PlayerMovementController
         }
 
         return new MovementResult(
-            Position: result.Position,
-            CellId: result.CellId,
-            IsOnGround: result.IsOnGround,
+            Position: Position,
+            CellId: CellId,
+            IsOnGround: !IsAirborne,
             MotionStateChanged: changed,
             ForwardCommand: forwardCmd,
             SidestepCommand: sidestepCmd,