M1c: weather, snap building, shelter, beds and regrowth

- Seeded weather (clear/cloudy/rain/storm, snow and blizzards on cold days
  and above the snowline), lightning, synthesised rain/wind/thunder sound
- Rain/snow particles stopped by roofs; wet ground, settled snow and wind
  via global shader uniforms
- Snap-together wood floor, wall, door, roof and gable with structural
  support, collapse and X to dismantle
- Shelter detection: dry under a roof, warmer and Rested inside with a fire;
  rain burns an uncovered campfire faster
- Bed sets the spawn point and lets you sleep through the night
- Trees, boulders and ground pickups regrow; timers saved
- Grass cleared under floors; shorter HUD hint
- New test_weather_building suite (62 checks) and tests/run_all.sh, which
  also fails on script errors

scripts/building/piece.gd

@@ -1,7 +1,8 @@
 extends StaticBody3D
-## Base for placed pieces (campfire, workbench, chest, later walls/floors).
-## Builds the visual from the item's procedural mesh and a box collider from
-## the item's footprint. Subclasses add behaviour.
+## Base for placed pieces (campfire, workbench, chest, bed, building pieces).
+## Builds the visual from the item's procedural mesh and, by default, a box
+## collider from the mesh bounds. Subclasses add behaviour or their own
+## colliders (_make_collision).
 
 var item_id := ""
 var def: Dictionary = {}
@@ -9,11 +10,19 @@ var def: Dictionary = {}
 func setup(d: Dictionary) -> void:
 	def = d
 	item_id = d.id
+	add_to_group("piece")
 	var mi := MeshInstance3D.new()
 	mi.name = "Mesh"
-	mi.mesh = ItemDB.mesh_for(item_id)
+	mi.mesh = _visual_mesh()
 	mi.material_override = ItemDB.item_material()
 	add_child(mi)
+	_make_collision(mi)
+	_build()
+
+func _visual_mesh() -> Mesh:
+	return ItemDB.mesh_for(item_id)
+
+func _make_collision(mi: MeshInstance3D) -> void:
 	var aabb := mi.mesh.get_aabb()
 	var cs := CollisionShape3D.new()
 	var box := BoxShape3D.new()
@@ -21,7 +30,6 @@ func setup(d: Dictionary) -> void:
 	cs.shape = box
 	cs.position = aabb.get_center()
 	add_child(cs)
-	_build()
 
 func _build() -> void:
 	pass
@@ -34,3 +42,7 @@ func interact_prompt(_player: Node) -> String:
 
 func interact(_player: Node) -> void:
 	pass
+
+## Whether X can take this piece back into your inventory (and why not).
+func dismantle_block_reason() -> String:
+	return ""