feat(composition): dive world assembled from a selected site definition

The calibration site spawn sat 16 m from the south bounds edge, inside the
40 m warning margin. The vehicle_test recipe now extends the terrain and
playable bounds 42 m to the south so the spawn is inside the zone.

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

content/sites/vehicle_test.tres

@@ -23,6 +23,6 @@ script = ExtResource("3_usfet")
 site_id = &"vehicle_test"
 display_name = "Site de calibration du véhicule"
 terrain = ExtResource("4_awe7g")
-playable_bounds_m = Rect2(-90, -176, 180, 204)
+playable_bounds_m = Rect2(-90, -176, 180, 246)
 spawn_position_m = Vector3(0, -12, 12)
 placements = Array[ExtResource("1_j2ii4")]([SubResource("Resource_5juth"), SubResource("Resource_tboiu")])