M1b: survival stats, day/night, fire fuel, cooking, foraging, torches, death + grave

- Health, stamina, hunger, warmth and wet with HUD bars, status tags and clock
- 20-minute day/night cycle with moon, stars and lit clouds
- Campfire fuel (2 min start, 15 min max), heat radius, cooking needs a lit fire
- Berry bushes (regrow 10 min), mushrooms, roasted mushroom, torch
- Stamina costs for sprint, jump and harvest hits
- Death respawns at island start and leaves a grave bag
- Saves include stats, time of day, fire fuel, regrowth timers and graves
- New test_survival_stats suite; tests fail if a script error aborts them

scripts/building/grave.gd

@@ -0,0 +1,23 @@
+extends "res://scripts/building/piece.gd"
+## Grave: left where you died, holding everything you carried. Open it with E
+## to take your things back; it crumbles once empty.
+
+var inventory := Inventory.new(40)
+
+func _build() -> void:
+	inventory.changed.connect(_on_changed)
+
+func display_name() -> String:
+	return "Your grave"
+
+func interact_prompt(_player: Node) -> String:
+	return "Your grave  [E] Recover belongings"
+
+func interact(player: Node) -> void:
+	player.open_inventory(self)
+
+func _on_changed() -> void:
+	for s in inventory.slots:
+		if s != null:
+			return
+	queue_free.call_deferred()