fix(arena): contain goal pockets

Game/scripts/goal.gd

@@ -14,6 +14,14 @@ signal goal_scored(team: int)
 # wall is thick or it pokes out the back of the arena. ArenaBoundary cuts the
 # matching aperture in the hull (see its GOAL_APERTURE_* constants).
 const POCKET_DEPTH := ArenaBoundary.SURFACE_THICKNESS
+# Physical back wall for the goal pocket. The arena end wall has a real hole
+# at the scoring aperture; without this, the visual-only net lets ships and
+# the ball leave the enclosure after crossing the sensor. A small overlap
+# into the surrounding end-wall panels closes numerical seams at the rim.
+const BACKSTOP_THICKNESS := 0.4
+const BACKSTOP_EDGE_OVERLAP := 0.25
+const ARENA_COLLISION_LAYER := 1 << 2
+const ARENA_COLLISION_MASK := (1 << 0) | (1 << 1)
 # Clearance between the mouth and the pocket shell, so the pocket's side walls
 # stay hidden behind the hull rather than showing at the aperture edge.
 const POCKET_CLEARANCE := 0.2
@@ -32,6 +40,7 @@ var _goal_burst: GPUParticles3D
 func _ready():
 	# Group lets AI controllers and game modes discover goals
 	add_to_group("goal")
+	_build_backstop()
 	# Everything below _on_body_entered is decoration, and training spawns
 	# headless arenas that never render it. The sensor is unaffected.
 	if DisplayServer.get_name() != "headless":
@@ -46,6 +55,26 @@ func _on_body_entered(body):
 		goal_scored.emit(team)
 
 
+func _build_backstop() -> void:
+	var mouth := ($CollisionShape3D.shape as BoxShape3D).size
+	var body := StaticBody3D.new()
+	body.name = "PocketBackstop"
+	body.collision_layer = ARENA_COLLISION_LAYER
+	body.collision_mask = ARENA_COLLISION_MASK
+	var collision := CollisionShape3D.new()
+	collision.name = "CollisionShape3D"
+	var box := BoxShape3D.new()
+	box.size = Vector3(
+		mouth.x + BACKSTOP_EDGE_OVERLAP * 2.0,
+		mouth.y + BACKSTOP_EDGE_OVERLAP * 2.0,
+		BACKSTOP_THICKNESS
+	)
+	collision.shape = box
+	collision.position = Vector3(0.0, 0.0, POCKET_DEPTH)
+	body.add_child(collision)
+	add_child(body)
+
+
 # --- visuals -----------------------------------------------------------------
 # Cosmetic only. The mouth is measured off the sensor's own collision shape, so
 # the frame can never drift from the volume that actually scores.