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/chest.gd

@@ -4,6 +4,12 @@ extends "res://scripts/building/piece.gd"
 const SLOTS := 16
 var inventory := Inventory.new(SLOTS)
 
+func dismantle_block_reason() -> String:
+	for s in inventory.slots:
+		if s != null:
+			return "Empty the chest first"
+	return ""
+
 func interact_prompt(_player: Node) -> String:
 	var used := 0
 	for s in inventory.slots: