M1a: survival core, pause menu, title screen and save/load

Design: docs/SURVIVAL_DESIGN.md (core loop, systems, architecture, phases).

Survival core (M1a):
- Items/recipes as data (data/*.json) in the addon SDK's define_item /
  define_recipe shape; procedural item meshes and offscreen-rendered icons
- Inventory class (40 slots, hotbar = row 1) shared by player and chests;
  click / right-click split / shift-click quick-move
- Actions autoload: the intent layer (craft, pickup, harvest_hit, place,
  transfer) that validates every change; becomes the RPC surface in M2
- Gathering: ground pickups (sticks, stones, flint); trees chopped with an
  axe fall and leave stumps; boulders mined with a pickaxe shrink per hit
- Crafting screen with station checks (workbench); placement mode with
  green/red ghost, rotation and overlap/slope validation
- Pieces: campfire (light, flames, embers), workbench, chest (16 slots)
- HUD: hotbar, held tool view model with swing, prompts, toasts

Menus and saving:
- ESC pause menu: resume, controls, fullscreen, mouse sensitivity, quit
- Title screen over a flyover of the island: Continue / New World (name +
  seed) / Load World (with delete) / Quit
- Saves autoload: one folder per world under user://worlds, atomic writes
  with a .bak; stores only player changes (player, pieces, chest contents,
  harvested resources, taken pickups) and regenerates the rest from the seed
- Autosave every 2 min, Save & Quit to title/desktop, save on window close

Fixes: UI screens now fill the viewport (were pinned top-left); title
backdrop no longer captures the mouse.

Tests: test_survival_core (31 checks), test_save_load (round trip + slots),
test_terrain_edges now checks 8 seeds for deep pits / unbanked water.

scripts/building/chest.gd

@@ -0,0 +1,15 @@
+extends "res://scripts/building/piece.gd"
+## Wooden chest: 16 slots of storage. E opens it next to your inventory.
+
+const SLOTS := 16
+var inventory := Inventory.new(SLOTS)
+
+func interact_prompt(_player: Node) -> String:
+	var used := 0
+	for s in inventory.slots:
+		if s != null:
+			used += 1
+	return "Chest (%d/%d)  [E] Open" % [used, SLOTS]
+
+func interact(player: Node) -> void:
+	player.open_inventory(self)