chore(sites): small cleanups and brief alignment

Drop a dead initial value in PlayableBoundsRule, guard
DiveSite.configure() when called before the node is in the tree, and
align the zone 1 brief with what was built: generic dive_site.tscn,
the resolved collision check, and the graybox shader deviation.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

game/application/sites/playable_bounds_rule.gd

@@ -29,7 +29,7 @@ func evaluate(position_m: Vector3) -> PlayableBoundsStatus:
 	var direction := Vector3.ZERO
 	if state != PlayableBoundsStatus.State.INSIDE:
 		var centre := _bounds_m.get_center()
-		var toward := Vector2(centre.x - point.x, centre.y - point.y)
+		var toward: Vector2
 		if distance == to_west or distance == to_east:
 			toward = Vector2(signf(centre.x - point.x), 0.0)
 		else: