fix(camera): retail-faithful jump-tracking via contact-plane projection

Original symptom: jumping made the camera swing around the player
vertically — the basis tilted up/down with the player's Z velocity.

Root cause: ComputeHeading used the raw 3D velocity vector as the
heading direction. During a jump, velocity has a substantial Z
component (vy ≈ jump speed), and `normalize((vx, vy, vz))` produced
a heading pointing up. The basis tilted accordingly and the camera
went under/over the player.

Retail's actual ALIGN_WITH_PLANE algorithm (decomp at
acclient_2013_pseudo_c.txt:95644-95795) is different:

  1. Velocity is only used as a gate. If |vx| AND |vy| > epsilon
     (player is moving in XY), proceed; otherwise fall back to the
     LOOK_IN_DIRECTION path (player's facing direction unchanged).
  2. The base heading is `localtoglobalvec(player, (0, 1, 0))` —
     the player's local +Y axis in world space, which in our
     convention is `(cos yaw, sin yaw, 0)`.
  3. Pick a surface normal:
       grounded:  contact_plane.N
       airborne:  (0, 0, 1)  [world up]
  4. Project the base heading onto the plane perpendicular to that
     normal:  projected = forward - normal * dot(forward, normal).
  5. Normalize. Fall back to the base if projection collapses.

Behaviorally:
  * Standing jump (vx≈0, vy≈0):  gate fails → base heading. Camera
    doesn't move with the jump.
  * Running jump (vx, vy, vz all nonzero, airborne):  projects onto
    world up → no-op since base is already horizontal. Camera basis
    stays horizontal; player visibly rises in frame.
  * Walking uphill (grounded, slope normal tilted):  projection
    adds a Z component matching the slope angle. Camera basis tilts
    with the terrain.
  * Walking on flat ground:  projection is a no-op. Camera basis
    horizontal.

Surface changes:
  * RetailChaseCamera.ComputeHeading gains `isOnGround` and
    `contactPlaneNormal` parameters.
  * RetailChaseCamera.Update gains the same two parameters and
    threads them through.
  * GameWindow's two Update call sites pass `result.IsOnGround` and
    `_playerController.ContactPlane.Normal` (already exposed on
    PlayerMovementController — no plumbing change there).
  * Tests: 2 existing heading tests reshaped (Moving* and Uphill);
    2 new tests added (AirborneJumping straight-up + running-jump);
    1 renamed (SlopeAlignDisabled). Net 25 → 27 tests in
    RetailChaseCameraTests; full AcDream.App.Tests: 39 → 41.

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -4757,7 +4757,11 @@ public sealed class GameWindow : IDisposable
 
                 // 4. Recenter chase camera on the new position.
                 _chaseCamera?.Update(snappedPos, _playerController.Yaw);
-                _retailChaseCamera?.Update(snappedPos, _playerController.Yaw, System.Numerics.Vector3.Zero, dt: 1f / 60f);
+                _retailChaseCamera?.Update(snappedPos, _playerController.Yaw,
+                    playerVelocity:     System.Numerics.Vector3.Zero,
+                    isOnGround:         true,
+                    contactPlaneNormal: System.Numerics.Vector3.UnitZ,
+                    dt:                 1f / 60f);
 
                 // 5. Return to InWorld.
                 _playerController.State = AcDream.App.Input.PlayerState.InWorld;
@@ -6429,10 +6433,18 @@ public sealed class GameWindow : IDisposable
             _chaseCamera.Update(result.RenderPosition, _playerController.Yaw,
                 isOnGround: result.IsOnGround,
                 dt: (float)dt);
-            // RetailChaseCamera: takes world velocity for slope-aligned heading;
-            // jump-feedback falls out of damping naturally, no isOnGround needed.
+            // RetailChaseCamera: heading is the player's facing direction
+            // projected onto the contact plane when grounded, or the
+            // world XY plane when airborne. The contact plane normal
+            // tilts the camera basis with terrain; the airborne
+            // fallback keeps the basis horizontal during jumps so the
+            // player visibly rises in frame without the camera
+            // swinging vertically (was the symptom of using raw
+            // velocity-vector heading).
             _retailChaseCamera!.Update(result.RenderPosition, _playerController.Yaw,
-                playerVelocity: _playerController.BodyVelocity,
+                playerVelocity:     _playerController.BodyVelocity,
+                isOnGround:         result.IsOnGround,
+                contactPlaneNormal: _playerController.ContactPlane.Normal,
                 dt: (float)dt);
 
             // Send outbound movement messages to the live server.