feat(*): add wall/ceiling surface pull and retune ball-ship materials

Game/scripts/ball.gd

@@ -0,0 +1,35 @@
+class_name Ball
+extends RigidBody3D
+
+# Physics ball. Floor gravity is untouched (gravity_scale in ball.tscn); this
+# only adds the wall/ceiling "surface pull" (see ArenaBoundary.get_surface_pull)
+# so the ball can cling near a wall for dribbling or hang against the ceiling
+# for ceiling shots, plus a safety speed clamp (the ball previously had none).
+
+@export_group("Surface Pull")
+@export var wall_pull_strength = 4.0     # Weaker than the ship's — assist, not adherence
+@export var wall_pull_range = 2.0
+@export var ceiling_pull_strength = 5.5  # Stays below the ball's effective gravity (0.8 * 9.8)
+@export var ceiling_pull_range = 2.0
+
+# Kept close to ship_observations.gd's BALL_SPEED_SCALE (30.0) so this feature
+# doesn't push ball velocity further out of the range trained policies expect.
+const MAX_SPEED := 32.0
+
+var _boundary: ArenaBoundary
+
+
+func _ready() -> void:
+	_boundary = get_tree().get_first_node_in_group("arena_boundary")
+
+
+func _integrate_forces(state: PhysicsDirectBodyState3D) -> void:
+	if _boundary:
+		var pull := _boundary.get_surface_pull(
+			global_position, wall_pull_strength, wall_pull_range,
+			ceiling_pull_strength, ceiling_pull_range
+		)
+		state.apply_central_force(pull * mass)
+
+	if state.linear_velocity.length() > MAX_SPEED:
+		state.linear_velocity = state.linear_velocity.normalized() * MAX_SPEED