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/ROADMAP.md

@@ -29,8 +29,10 @@ Detailed plan in `docs/SURVIVAL_DESIGN.md` (phases M1a–M1d).
 - [x] Crafting (recipe: wood → tools / building pieces) *(M1a)*
 - [x] Pause menu, title screen, world slots, save/load, autosave *(M1a)*
 - [x] Health / stamina / hunger / warmth, day-night cycle, fire fuel, cooking, foraging, torches, death + grave *(M1b)*
-- [ ] Weather (rain, storm, snow), wet/shelter *(M1c)*
-- [ ] Snap-together building: place a wall/floor, resource cost, basic support rule
+- [x] Weather (rain, storm, snow), wet/shelter *(M1c)*
+- [x] Snap-together building: floor/wall/door/roof/gable, resource cost, support + collapse, dismantling *(M1c)*
+- [x] Bed spawn point and sleeping, resource regrowth *(M1c)*
+- [ ] Creatures, hunting, weapons *(M1d)*
 - **Done when:** gather → craft → build a tiny shelter, single-player.
 
 ## M2 — Multiplayer · ~2–4 months