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/world/pickup.gd

@@ -30,6 +30,18 @@ func interact_prompt(_player: Node) -> String:
 func interact(player: Node) -> void:
 	Actions.pickup(player, self)
 
+signal taken_now(pickup: Node)
+
+## Picked up: hidden (not freed) so it can reappear later (props.gd regrowth).
 func collected() -> void:
+	if taken:
+		return
 	taken = true
-	queue_free()
+	visible = false
+	collision_layer = 0
+	taken_now.emit(self)
+
+func regrow() -> void:
+	taken = false
+	visible = true
+	collision_layer = 2