feat: vector-based movement for responsive control and air strafing

Changed from physics-based (accumulate + damping) to vector-based:

- Horizontal velocity is SET directly, not accumulated
  - Press A → cvx = -0.5 (instant, no ramp-up)
  - Release → cvx = 0 (instant stop, no sliding)
  - No horizontal damping = precise control
- Air strafing: same horizontal speed in air as on ground
  - Can change direction mid-jump freely
- Vertical: gravity + minimal damping (0.99) for natural fall
  - Jump sets cvy directly, gravity pulls back
- move_speed 0.5 (was 0.3), max_vel 2.0 (was 1.0)
- Player: move_dir tracks current direction, stop_horizontal() clears velocity
- handle_input: calls stop_horizontal when no movement key held

109 tests, 0 warnings, 0 failures

src/physics/verlet.rs

@@ -97,7 +97,7 @@ impl VerletSolver {
             gravity: 0.04,
             damping: 0.97,
             dt: 1.0,
-            max_vel: 1.0,
+            max_vel: 2.0,
             substeps: 8,
         }
     }