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

docs/SETUP_ON_NEW_MACHINE.md

@@ -14,7 +14,14 @@ download — the M0 world is generated in code.
   `/Applications`.
 
 ## Tests
+Run everything (fails on any failed check or script error):
 ```bash
+tests/run_all.sh
+```
+Or one at a time:
+```bash
+# Weather, shelter, snap building, support, doors, beds, regrowth
+godot --headless --path . --script res://tests/test_weather_building.gd
 # Boundary: tries to walk/jump out of the valley in 24 directions
 godot --headless --path . --fixed-fps 30 --script res://tests/test_boundary.gd
 # Terrain: no deep pits / floating water at the boundary, across 8 seeds