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>

game/composition/vertical_slice/dive_world.tscn

@@ -0,0 +1,43 @@
+[gd_scene load_steps=7 format=3]
+
+[ext_resource type="PackedScene" path="res://game/world/sites/dive_site.tscn" id="1_site"]
+[ext_resource type="PackedScene" path="res://game/world/submarine/submarine_actor.tscn" id="2_submarine"]
+[ext_resource type="PackedScene" path="res://game/presentation/cockpit/cockpit_camera_rig.tscn" id="3_camera"]
+[ext_resource type="PackedScene" path="res://game/presentation/cockpit/cockpit_interior.tscn" id="4_cockpit"]
+
+[sub_resource type="ParticleProcessMaterial" id="SuspendedMotion"]
+emission_shape = 3
+emission_box_extents = Vector3(45, 28, 70)
+direction = Vector3(0.1, 0.04, 0)
+spread = 180.0
+initial_velocity_min = 0.02
+initial_velocity_max = 0.09
+gravity = Vector3(0, 0, 0)
+scale_min = 0.025
+scale_max = 0.08
+
+[sub_resource type="SphereMesh" id="SuspendedParticleMesh"]
+radius = 0.035
+height = 0.07
+
+[node name="DiveWorld" type="Node3D"]
+
+[node name="DiveSite" parent="." instance=ExtResource("1_site")]
+unique_name_in_owner = true
+
+[node name="SubmarineActor" parent="." instance=ExtResource("2_submarine")]
+unique_name_in_owner = true
+
+[node name="SuspendedParticles" type="GPUParticles3D" parent="SubmarineActor"]
+amount = 360
+lifetime = 14.0
+preprocess = 14.0
+visibility_aabb = AABB(-50, -35, -78, 100, 70, 156)
+process_material = SubResource("SuspendedMotion")
+draw_pass_1 = SubResource("SuspendedParticleMesh")
+
+[node name="CockpitInterior" parent="SubmarineActor" instance=ExtResource("4_cockpit")]
+unique_name_in_owner = true
+
+[node name="CockpitCameraRig" parent="SubmarineActor" instance=ExtResource("3_camera")]
+unique_name_in_owner = true