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

shaders/rock.gdshader

@@ -33,8 +33,13 @@ void fragment() {
 	float snow = smoothstep(snow_height - 3.0, snow_height + 3.0, w_pos.y + (a - 0.5) * 10.0);
 	snow *= smoothstep(0.35, 0.7, w_nrm.y);
 	col = mix(col, snow_col, snow);
+	float wsnow = snow_mask(w_nrm.y, a);
+	col = mix(col, snow_col, wsnow);
+	snow = max(snow, wsnow);
+	float wet = weather_wet * (1.0 - snow);
+	col = wet_albedo(col, wet);
 
 	ALBEDO = col;
-	ROUGHNESS = mix(mix(0.8, 0.95, moss), 0.6, snow);
+	ROUGHNESS = mix(mix(mix(0.8, 0.95, moss), 0.6, snow), 0.3, wet * 0.8);
 	SPECULAR = 0.35;
 }