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.

docs/SETUP_ON_NEW_MACHINE.md

@@ -17,10 +17,20 @@ download — the M0 world is generated in code.
 ```bash
 # Boundary: tries to walk/jump out of the valley in 24 directions
 godot --headless --path . --fixed-fps 30 --script res://tests/test_boundary.gd
-# Terrain: no pits/hanging water just inside the boundary wall
+# Terrain: no deep pits / floating water at the boundary, across 8 seeds
 godot --headless --path . --script res://tests/test_terrain_edges.gd
+# Survival loop: inventory, crafting, chopping, mining, pickups, building, chests
+godot --headless --path . --script res://tests/test_survival_core.gd
+# Save/load round trip + world slots (uses user://test_worlds, not your saves)
+godot --headless --path . --script res://tests/test_save_load.gd
 ```
 
+## Save files
+Worlds are per machine, under
+`~/Library/Application Support/Godot/app_userdata/Valheimlike/worlds/` on
+macOS (`%APPDATA%\Godot\app_userdata\Valheimlike\worlds\` on Windows). Copy a
+world's folder across to move it to another machine.
+
 ## 2. Get the project onto the machine
 You have the folder (from the zip, or a git remote). Then: