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

scripts/world/day_night.gd

@@ -28,6 +28,11 @@ var _sky_timer := 0.0
 const DAY_COLS := [Color(0.24, 0.44, 0.72), Color(0.72, 0.79, 0.85), Color(0.70, 0.77, 0.85)]
 const DUSK_COLS := [Color(0.22, 0.27, 0.48), Color(0.98, 0.62, 0.38), Color(0.82, 0.62, 0.50)]
 const NIGHT_COLS := [Color(0.02, 0.03, 0.07), Color(0.07, 0.09, 0.16), Color(0.07, 0.09, 0.14)]
+const OVERCAST_COLS := [Color(0.40, 0.44, 0.49), Color(0.58, 0.61, 0.64), Color(0.56, 0.59, 0.62)]
+const BASE_FOG := 0.0035          ## default_env.tres fog_density
+
+var weather: Node                  ## weather.gd, set by world.gd
+var _flash_shown := false
 
 func setup(world_sun: DirectionalLight3D, environment: Environment) -> void:
 	sun = world_sun
@@ -78,37 +83,54 @@ func apply(force: bool) -> void:
 	daylight = smoothstep(-0.2, 0.25, to_sun.y)
 	var low := 1.0 - smoothstep(0.05, 0.45, absf(to_sun.y))   # 1 near the horizon
 
+	# Weather (weather.gd) dims the sun, thickens fog and greys the sky.
+	var w_light := 1.0
+	var w_fog := 1.0
+	var w_clouds := 0.3
+	var w_flash := 0.0
+	if weather != null:
+		w_light = weather.light
+		w_fog = weather.fog
+		w_clouds = weather.clouds
+		w_flash = weather.flash
+	var overcast := smoothstep(0.5, 1.0, w_clouds)
+
 	sun.global_basis = Basis.looking_at(-to_sun if to_sun.y > -0.2 else Vector3.DOWN, Vector3.FORWARD)
-	sun.light_energy = 1.8 * smoothstep(-0.03, 0.15, to_sun.y)
+	sun.light_energy = 1.8 * smoothstep(-0.03, 0.15, to_sun.y) * w_light
 	sun.light_color = Color(1.0, 0.95, 0.83).lerp(Color(1.0, 0.58, 0.32), low)
 	sun.visible = sun.light_energy > 0.01
 	sun.shadow_enabled = sun.visible
 
 	var moon_dir := Vector3(-to_sun.x, absf(to_sun.y) * 0.8 + 0.25, -to_sun.z * 0.5).normalized()
 	moon.global_basis = Basis.looking_at(-moon_dir, Vector3.FORWARD)
-	moon.light_energy = 0.38 * (1.0 - daylight)
+	moon.light_energy = 0.38 * (1.0 - daylight) * (1.0 - 0.6 * overcast)
 	moon.visible = moon.light_energy > 0.01
 	moon.shadow_enabled = moon.visible and not sun.visible
 
-	env.ambient_light_energy = lerpf(0.3, 1.0, daylight)
-	env.fog_light_energy = lerpf(0.25, 1.0, daylight)
-	env.volumetric_fog_density = lerpf(0.0025, 0.0045, daylight)
+	env.ambient_light_energy = lerpf(0.3, 1.0, daylight) * lerpf(1.0, 0.85, overcast) + w_flash * 3.0
+	env.fog_light_energy = lerpf(0.25, 1.0, daylight) * lerpf(1.0, 0.7, overcast) + w_flash * 2.0
+	env.volumetric_fog_density = lerpf(0.0025, 0.0045, daylight) * lerpf(1.0, 2.0, overcast)
+	env.fog_density = BASE_FOG * w_fog
 
 	_sky_timer -= get_process_delta_time()
-	if not force and _sky_timer > 0.0:
+	if not force and _sky_timer > 0.0 and w_flash < 0.05 and not _flash_shown:
 		return
+	_flash_shown = w_flash >= 0.05
 	_sky_timer = 0.25
 	var cols: Array = []
 	for i in range(3):
-		var dayc: Color = DAY_COLS[i].lerp(DUSK_COLS[i], low * daylight)
+		var dayc: Color = DAY_COLS[i].lerp(DUSK_COLS[i], low * daylight * (1.0 - overcast * 0.7))
+		dayc = dayc.lerp(OVERCAST_COLS[i], overcast * 0.85)
 		cols.append(NIGHT_COLS[i].lerp(dayc, daylight))
 	env.fog_light_color = cols[2]
 	if sky_mat != null:
 		sky_mat.set_shader_parameter("zenith_col", cols[0])
 		sky_mat.set_shader_parameter("horizon_col", cols[1])
-		sky_mat.set_shader_parameter("energy", lerpf(0.35, 1.0, daylight))
-		sky_mat.set_shader_parameter("star_strength", 1.0 - smoothstep(0.0, 0.35, daylight))
-		sky_mat.set_shader_parameter("cloud_lit", Color(0.16, 0.18, 0.24).lerp(Color(1, 1, 1), daylight))
+		sky_mat.set_shader_parameter("energy", lerpf(0.35, 1.0, daylight) * lerpf(1.0, 0.72, overcast) + w_flash * 2.5)
+		sky_mat.set_shader_parameter("star_strength", (1.0 - smoothstep(0.0, 0.35, daylight)) * (1.0 - overcast))
+		sky_mat.set_shader_parameter("cloud_cover", lerpf(0.35, 1.05, w_clouds))
+		var lit := Color(0.16, 0.18, 0.24).lerp(Color(1, 1, 1), daylight)
+		sky_mat.set_shader_parameter("cloud_lit", lit.lerp(lit * Color(0.55, 0.57, 0.6), overcast))
 		sky_mat.set_shader_parameter("sun_tint", Color(1.0, 0.86, 0.62).lerp(Color(1.0, 0.55, 0.3), low))
 
 ## 0..100 "how warm the air is" before fires, shelter and clothes.