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

tests/test_survival_stats.gd

@@ -61,6 +61,7 @@ func _body(world: Node3D, id: String) -> void:
 	var terrain: Node3D = world.get_node("Terrain")
 	player.set_physics_process(false)   # keep the player put while we fast-forward
 	sv.set_process(false)
+	world.weather.force("clear")         # weather has its own test (test_weather_building)
 
 	print("day / night")
 	check(dn.day == 1 and absf(dn.hours() - 8.0) < 0.1, "new world starts on day 1 at 08:00 (%s)" % dn.clock_text())