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/SURVIVAL_DESIGN.md

@@ -55,7 +55,13 @@ Item fields: `id`, `name`, `description`, `stack`, `category`
 | Animals *(M1d)* | weapon | raw meat, hide, bone |
 
 Tool **tiers** gate harder resources later (e.g. iron ore needs a tier-2
-pickaxe). Felled trees leave stumps. Resources regrow over time *(M1c)*.
+pickaxe). Felled trees leave stumps.
+
+**Regrowth** *(built in M1c, `props.gd`)*: felled trees grow back after
+2400 s (2 days), mined boulders after 3600 s, picked-up sticks/stones/flint/
+mushrooms after 900 s. A timer waits (retries every 2 min) while you're
+standing on the spot or something is built there. Timers are saved, and
+sleeping advances them.
 
 ### 2.4 Crafting & stations
 Recipes list inputs, an output and an optional **station** that must be within
@@ -65,12 +71,34 @@ panel shows every recipe you know, greyed out with the missing inputs in red.
 ### 2.5 Building
 - **Placement mode**: holding a placeable shows a ghost preview (green = valid,
   red = blocked). R rotates it, left-click places it.
-- M1a pieces: campfire, workbench, chest.
-- *(M1c)* Snap-together pieces: wood floor, wall, door, roof, stairs, with
-  snap points and Valheim-style **structural support** (stability falls off
-  with distance from the ground; unsupported pieces collapse).
-- **Shelter detection**: you count as sheltered when rays upward hit a roof and
-  most sideways rays hit walls. That drives "dry" and "rested" status.
+- M1a pieces: campfire, workbench, chest. M1c adds a bed.
+- *(built in M1c)* Snap-together wood pieces, crafted at a workbench
+  (shift-click Craft makes 5): floor (2 wood), wall (2), door (4), roof
+  (2 wood + 1 fiber), gable (1). Everything is 2 m on a side. A roof rises
+  1 m over 2 m, so two roofs from opposite walls of a 4 m hut meet at a ridge,
+  and gables close the ends. Geometry and snap points live in
+  `build_piece.gd`, rules in `build_system.gd`.
+- **Snapping**: the ghost sits on your crosshair, then moves to the nearest
+  pair of snap points (its own and a nearby piece's), with rotation locked to
+  that piece's 90-degree grid. R turns building pieces a quarter turn. Away
+  from other pieces, floors sit level on the highest ground under them.
+- **Support**: touching the ground = 100%. Otherwise a piece takes the best
+  of its neighbours minus 10% going up/down or 25% sideways. Under 30% it
+  can't be placed, so an overhang reaches about 4 floors.
+- **Dismantling**: X on any placed piece puts it back in your inventory
+  (chests must be empty, graves can't be). Pieces left below 30% support
+  collapse and come back to you too. Loose chests/fires/beds drop to the
+  ground.
+- **Shelter detection**: roofed = 3 of 5 rays straight up hit a building piece.
+  Sheltered = roofed and at least 6 of 8 sideways rays hit walls within 8 m.
+  Roofed keeps rain off you (and off a campfire). Sheltered cuts the weather's
+  chill to a quarter, adds +8 warmth, and with a fire nearby makes you Rested
+  after 20 s (stamina regen +50% for 5 min).
+- **Bed**: E sets your spawn point (death respawns you there while it
+  stands). E again at night (19:30 to 05:30) sleeps until 06:00 if the bed is
+  sheltered: the clock, fires, regrowth and weather move on, you lose a
+  little food, and you wake dry, warm and Rested.
+- Grass doesn't grow through floors.
 
 ### 2.6 Survival stats *(M1b)*
 | Stat | Drains from | Restored by | At zero |
@@ -103,10 +131,26 @@ Built in M1b (`scripts/player/survival.gd`), current numbers:
 - Day/night cycle: 20 real minutes per day by default. The sun, sky, fog and
   ambient light follow time of day. Nights are dark enough that a campfire or
   torch matters.
-- Weather is a seeded state machine: clear → cloudy → rain → storm, plus
-  **snow** on cold days and always near the snowline. Each state sets the
-  cloud cover, fog, wind (grass/trees already read wind), particles and
-  sound, plus a warmth modifier. Transitions blend over about a minute.
+- *(built in M1c)* Weather is a function of the world seed and the absolute
+  world time (`weather.gd`), so it isn't saved and every M2 client can
+  compute the same weather. A slow noise "front" picks the state. Measured
+  over 60 days on seed 1337: clear 38%, cloudy about 30%, rain/snow 33%,
+  storm 12%. A wet spell lasts about 6 minutes. A new world's first
+  10 minutes are always clear.
+- About 1 day in 4 (never day 1) is a **cold day** (-8 warmth): rain falls as
+  snow and storms become blizzards. Above the snowline (28 m) it always
+  snows.
+- Warmth from weather: cloudy -3, rain -6, storm -12, snow -14, blizzard -24.
+  Rain on you (no roof) keeps you Wet. Snow chills but doesn't soak. Rain on
+  an uncovered campfire burns its fuel up to 4x faster.
+- Each state sets cloud cover, sun and ambient light, fog, wind and
+  precipitation, blending over about 50 s. Storms flash lightning, with
+  thunder 0.5 to 2.5 s later.
+- Visuals and sound (`weather_fx.gd`, client side): GPU rain and snow
+  particles stopped by a heightfield collider, so it doesn't rain indoors.
+  Wet ground, settled snow (on terrain, rocks, grass and tree crowns) and wind
+  strength are global shader uniforms. Rain, wind and thunder sounds are
+  synthesised from filtered noise, and rain is muffled under a roof.
 
 ### 2.8 Creatures & hunting *(M1d)*
 - **Rabbit**: skittish, fast, gives a little meat.
@@ -137,7 +181,9 @@ Built in M1b (`scripts/player/survival.gd`), current numbers:
   inventory, hotbar slot; placed pieces and chest contents; tree and boulder
   damage, stumps, mined rocks; collected pickups. Everything else regenerates
   from the seed. Since M1b it also stores stats, time of day, bush regrowth
-  timers, campfire fuel and graves. *(Later: weather.)*
+  timers, campfire fuel and graves. Since M1c: building pieces (door open or
+  shut), the bed spawn point and regrowth timers. Weather isn't saved: it
+  follows from the seed and the time.
 
 ### 2.11 Death *(built in M1b)*
 - On death you respawn at your bed (or the island spawn) and leave a **grave
@@ -163,7 +209,7 @@ Built in M1b (`scripts/player/survival.gd`), current numbers:
 |---|---|---|
 | **M1a** *(done)* | items/recipes data, inventory + hotbar, pickups, chop/mine with tools, crafting UI, campfire / workbench / chest placement, chest storage, pause menu, **title screen + world slots + save/load + autosave** (pulled forward from M1c) | you craft an axe, fell a tree, build a fire, stash wood in a chest, quit and continue |
 | **M1b** *(done)* | hunger/warmth/stamina/health HUD, day/night cycle with moon and stars, campfire warmth + fuel, cooking, bushes (fiber, berries), mushrooms, torches, **death + grave bag** (pulled forward from M1c) | you survive your first night by the fire |
-| **M1c** | weather (rain, storm, snow) + wet/shelter, snap building (floor/wall/roof/door) with support, bed spawn point, resource regrowth | you build a hut and wait out a storm |
+| **M1c** *(done)* | weather (rain, storm, snow, blizzard, lightning, sound) + wet/shelter/rested, snap building (floor/wall/door/roof/gable) with support, dismantling and collapse, bed spawn point + sleeping, resource regrowth | you build a hut and wait out a storm |
 | **M1d** | rabbit/deer/boar AI, club/spear/bow, hides, cooked food buffs | you hunt a deer and eat its meat |
 
 ## 5. Open decisions (current defaults in bold)