refactor(physics): wire PhysicsBody + MotionInterpreter into PlayerMovementController

Replace the ad-hoc movement simulation with the ported retail physics:

- PlayerMovementController now owns a PhysicsBody (gravity, friction, Euler
  integration with sub-stepping) and a MotionInterpreter (motion state machine,
  speed constants from retail dat).

- Orientation quaternion is synced from Yaw each frame (Yaw=0 → +X, matching
  the cos/sin convention the camera and outbound messages expect).

- Horizontal velocity is composed from MotionInterpreter.get_state_velocity()
  speeds (WalkAnimSpeed=3.12, RunAnimSpeed=4.0, SidestepAnimSpeed=1.25 from
  decompiled globals) then pushed via PhysicsBody.set_local_velocity so the
  orientation quaternion rotates them into world space correctly.

- Vertical velocity (gravity / jump / fall) is snapshot before DoMotion calls
  so apply_current_movement's set_local_velocity(0,0,0) can't clobber it.

- Jump delegates to MotionInterpreter.jump() + LeaveGround() which calls
  get_leave_ground_velocity() → DefaultJumpVz=10.0 (retail value).

- PhysicsEngine.Resolve is still called each frame with zero delta to sample
  terrain/cell Z under the body and set Contact+OnWalkable accordingly.

- Drive UpdatePhysicsInternal(dt) directly instead of update_object(wallClock)
  to avoid the MinQuantum (~33ms) guard that would silently drop 60fps frames.

Test update: jump loop extended from 30→50 frames to cover the longer flight
time from retail DefaultJumpVz=10 (≈2.04s) vs old JumpImpulse=5 (≈1.02s).

303 tests green.

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

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

@@ -131,12 +131,13 @@ public class PlayerMovementControllerTests
         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++)
+        // Many frames — should come back down.
+        // DefaultJumpVz = 10 m/s → full flight time ≈ 2.04s, so run 50 × 50ms = 2.5s
+        // to ensure the player has definitely landed.
+        for (int i = 0; i < 50; i++)
             controller.Update(0.05f, new MovementInput());
 
         Assert.False(controller.IsAirborne, "Should have landed");
-        // +0.15 Z bias keeps feet above terrain surface (prevents z-fighting).
         Assert.Equal(50f, controller.Position.Z, precision: 1);
     }