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/common.gdshaderinc

@@ -5,6 +5,22 @@
 
 uniform sampler2D noise_tex : filter_linear_mipmap, repeat_enable;
 
+// Set every frame by weather_fx.gd (declared in project.godot [shader_globals]).
+global uniform float weather_wet;    // 0..1 rain-soaked ground
+global uniform float weather_snow;   // 0..1 settled snow
+global uniform float weather_wind;   // wind multiplier, ~1 on a calm day
+
+// Wet surfaces: darker and glossier.
+vec3 wet_albedo(vec3 c, float wet) {
+	return c * (1.0 - 0.35 * wet);
+}
+
+// Settled snow on upward-facing surfaces, patchy where it's thin.
+float snow_mask(float up, float patch) {
+	float s = weather_snow * 1.25 - (1.0 - patch) * 0.45;
+	return clamp(s, 0.0, 1.0) * smoothstep(0.55, 0.85, up);
+}
+
 float nz(vec2 p) {
 	return texture(noise_tex, p).r;
 }