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/bed.gd

@@ -0,0 +1,18 @@
+extends "res://scripts/building/piece.gd"
+## Bed: E sets your spawn point here. Once it's yours, E at night sleeps until
+## morning, if the bed is under a roof with walls around it.
+
+func display_name() -> String:
+	return "Bed"
+
+func is_spawn() -> bool:
+	var w := get_parent().get_parent()
+	return w.get("bed_spawn") != null and w.has_bed_spawn and (w.bed_spawn as Vector3).distance_to(global_position) < 0.3
+
+func interact_prompt(_player: Node) -> String:
+	if not is_spawn():
+		return "Bed  [E] Set spawn point"
+	return "Bed (your spawn point)  [E] Sleep"
+
+func interact(player: Node) -> void:
+	Actions.use_bed(player, self)