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.

tests/test_terrain_edges.gd

@@ -1,19 +1,39 @@
 extends SceneTree
-## Terrain edge check: there must be no ground below the inland waterline in the
-## band just inside the boundary wall. Ground like that shows up as inland water
-## hanging over a drop (the lake water surface stops at the wall) and makes pits
-## the player can fall into and not climb out of.
+## Terrain edge check, for several seeds: in the band just inside the boundary
+## wall there must be (1) no water deeper than you can wade out of, and (2) no
+## water without a bank, i.e. ground that drops away outward. The inland water
+## surface stops at the wall, so unbanked water hangs in the air over the drop.
+## This is the bug where the player fell into floating water by the sea cliff.
+## Shallow ponds at the foot of the mountains are fine: the slope holds them.
 ##
 ## Run: godot --headless --path . --script res://tests/test_terrain_edges.gd
 
-const BAND := 10.0   # metres inside the wall to check
+const BAND := 10.0            # metres inside the wall to check
+const MAX_EDGE_DEPTH := 1.0   # deepest allowed water near the edge (wadeable)
+const BANK_CHECK := 12.0      # how far outward the bank must rise above water
+
+## The original island plus a spread of other seeds: new worlds get random
+## seeds, so the edge rule has to hold for any of them.
+const SEEDS := [1337, 1, 42, 777, 4242, 9001, 31337, 123456]
 
 func _initialize() -> void:
-	var terrain: Node3D = load("res://scripts/world/terrain.gd").new()
-	root.add_child(terrain)   # builds in _ready
-	_run.call_deferred(terrain)
+	_all.call_deferred()
 
-func _run(terrain: Node3D) -> void:
+func _all() -> void:
+	var failed := 0
+	for s in SEEDS:
+		var terrain: Node3D = load("res://scripts/world/terrain.gd").new()
+		terrain.noise_seed = s
+		root.add_child(terrain)   # builds in _ready
+		var bad := _check(terrain)
+		print("seed %6d: %s" % [s, "ok" if bad == 0 else "%d bad samples" % bad])
+		if bad > 0:
+			failed += 1
+		terrain.free()
+	print("RESULT: %s (%d/%d seeds clean)" % ["PASS" if failed == 0 else "FAIL", SEEDS.size() - failed, SEEDS.size()])
+	quit(1 if failed > 0 else 0)
+
+func _check(terrain: Node3D) -> int:
 	var bad := 0
 	var worst := INF
 	var worst_at := Vector2.ZERO
@@ -24,7 +44,16 @@ func _run(terrain: Node3D) -> void:
 		while z < half:
 			if terrain.inside_boundary(x, z, 0.0) and not terrain.inside_boundary(x, z, BAND):
 				var h: float = terrain.sample_height(x, z)
-				if h < terrain.water_level + 0.2:
+				# Hazard 1: a pit too deep to wade out of near the edge.
+				var too_deep: bool = h < terrain.water_level - MAX_EDGE_DEPTH
+				# Hazard 2: water that isn't held in by a bank. The inland water
+				# surface stops at the wall, so wet ground must rise again just
+				# outward, or the water edge hangs over a drop.
+				var unbanked := false
+				if h < terrain.water_level:
+					var out := Vector2(x, z).normalized() * BANK_CHECK
+					unbanked = terrain.sample_height(x + out.x, z + out.y) < terrain.water_level
+				if too_deep or unbanked:
 					bad += 1
 					if h < worst:
 						worst = h
@@ -32,8 +61,6 @@ func _run(terrain: Node3D) -> void:
 			z += 1.0
 		x += 1.0
 	if bad > 0:
-		print("FAIL: %d samples near the wall are below the waterline; lowest %.1f m at (%.0f, %.0f), sea weight %.2f" % [
-			bad, worst, worst_at.x, worst_at.y, terrain.sea_weight(worst_at.x, worst_at.y)])
-	else:
-		print("PASS: ground near the wall stays above the waterline")
-	quit(1 if bad > 0 else 0)
+		print("  lowest %.1f m at (%.0f, %.0f), sea weight %.2f" % [
+			worst, worst_at.x, worst_at.y, terrain.sea_weight(worst_at.x, worst_at.y)])
+	return bad