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/world/sites/dive_site.gd

@@ -19,6 +19,8 @@ var _placeholder_count := 0
 
 
 func configure(definition: DiveSiteDefinition) -> String:
+	if _terrain == null or _placements_root == null:
+		return "dive site is not in the scene tree"
 	if definition == null:
 		return "dive site definition is missing"
 	var field := definition.height_field()