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/core/item_db.gd

@@ -0,0 +1,135 @@
+extends Node
+## Autoload "ItemDB": item and recipe definitions loaded from data/*.json, plus
+## procedurally rendered inventory icons.
+##
+## Definitions use the same shape as the addon SDK's define_item /
+## define_recipe (docs/ADDON_SDK.md), so addon content will later be
+## registered through `define_item()` / `define_recipe()` like the base game.
+
+signal icons_ready
+
+const ItemMeshes = preload("res://scripts/items/item_meshes.gd")
+const ICON_SIZE := 96
+
+var items: Dictionary = {}     # id -> definition Dictionary
+var recipes: Dictionary = {}   # id -> recipe Dictionary (insertion order kept)
+var icons: Dictionary = {}     # id -> Texture2D
+var _item_mat: StandardMaterial3D
+
+func _ready() -> void:
+	_load("res://data/items.json", "items", define_item)
+	_load("res://data/recipes.json", "recipes", define_recipe)
+	if DisplayServer.get_name() != "headless":
+		_render_icons.call_deferred()
+
+func _load(path: String, key: String, define: Callable) -> void:
+	var text := FileAccess.get_file_as_string(path)
+	var data = JSON.parse_string(text)
+	if typeof(data) != TYPE_DICTIONARY or not data.has(key):
+		push_error("ItemDB: could not parse %s" % path)
+		return
+	for d in data[key]:
+		define.call(d)
+
+func define_item(d: Dictionary) -> void:
+	for f in ["id", "name", "category", "stack"]:
+		if not d.has(f):
+			push_error("ItemDB: item missing '%s': %s" % [f, d])
+			return
+	d.stack = int(d.stack)
+	items[d.id] = d
+
+func define_recipe(r: Dictionary) -> void:
+	if not items.has(r.get("output", "")):
+		push_error("ItemDB: recipe '%s' outputs unknown item" % r.get("id", "?"))
+		return
+	for id in r.inputs:
+		if not items.has(id):
+			push_error("ItemDB: recipe '%s' uses unknown item '%s'" % [r.id, id])
+			return
+	r.count = int(r.get("count", 1))
+	r.station = String(r.get("station", ""))
+	recipes[r.id] = r
+
+func get_item(id: String) -> Dictionary:
+	return items.get(id, {})
+
+func item_name(id: String) -> String:
+	return items.get(id, {}).get("name", id)
+
+func stack_size(id: String) -> int:
+	return int(items.get(id, {}).get("stack", 1))
+
+func mesh_for(id: String) -> ArrayMesh:
+	return ItemMeshes.build(items.get(id, {}).get("mesh", id))
+
+## Shared material for item meshes (vertex colours, authored as sRGB).
+func item_material() -> StandardMaterial3D:
+	if _item_mat == null:
+		_item_mat = StandardMaterial3D.new()
+		_item_mat.vertex_color_use_as_albedo = true
+		_item_mat.vertex_color_is_srgb = true
+		_item_mat.roughness = 0.85
+	return _item_mat
+
+func icon(id: String) -> Texture2D:
+	return icons.get(id, null)
+
+# --- icon renderer ----------------------------------------------------------
+
+## Renders every item mesh once in an offscreen viewport and keeps the images.
+func _render_icons() -> void:
+	var vp := SubViewport.new()
+	vp.size = Vector2i(ICON_SIZE, ICON_SIZE)
+	vp.transparent_bg = true
+	vp.own_world_3d = true
+	vp.msaa_3d = Viewport.MSAA_4X
+	vp.render_target_update_mode = SubViewport.UPDATE_ALWAYS
+	add_child(vp)
+
+	var env := Environment.new()
+	env.background_mode = Environment.BG_CLEAR_COLOR
+	env.ambient_light_source = Environment.AMBIENT_SOURCE_COLOR
+	env.ambient_light_color = Color(0.75, 0.78, 0.85)
+	env.ambient_light_energy = 0.7
+	env.tonemap_mode = Environment.TONE_MAPPER_FILMIC
+	var we := WorldEnvironment.new()
+	we.environment = env
+	vp.add_child(we)
+
+	var key := DirectionalLight3D.new()
+	key.light_energy = 1.6
+	key.rotation = Vector3(deg_to_rad(-50), deg_to_rad(-35), 0)
+	vp.add_child(key)
+	var fill := DirectionalLight3D.new()
+	fill.light_energy = 0.5
+	fill.light_color = Color(0.7, 0.8, 1.0)
+	fill.rotation = Vector3(deg_to_rad(-20), deg_to_rad(140), 0)
+	vp.add_child(fill)
+
+	var cam := Camera3D.new()
+	cam.projection = Camera3D.PROJECTION_ORTHOGONAL
+	vp.add_child(cam)
+	var mi := MeshInstance3D.new()
+	mi.material_override = item_material()
+	vp.add_child(mi)
+
+	for id in items:
+		var mesh := mesh_for(id)
+		mi.mesh = mesh
+		# Tools are long and thin: tilt them diagonally across the icon.
+		var cat: String = items[id].category
+		mi.rotation = Vector3(0, 0, deg_to_rad(-45)) if cat in ["tool", "weapon"] else Vector3(0, deg_to_rad(30), 0)
+		var aabb := mi.transform * mesh.get_aabb()
+		var c := aabb.get_center()
+		var extent := maxf(aabb.size.length(), 0.05)
+		cam.size = extent * 0.95
+		cam.position = c + Vector3(0, 0.55, 1).normalized() * extent * 2.0
+		cam.look_at(c, Vector3.UP)
+		cam.near = 0.01
+		cam.far = extent * 5.0
+		await RenderingServer.frame_post_draw
+		await RenderingServer.frame_post_draw
+		icons[id] = ImageTexture.create_from_image(vp.get_texture().get_image())
+	vp.queue_free()
+	icons_ready.emit()