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/player/player.gd

@@ -18,19 +18,46 @@ const FLOAT_DEPTH := 1.3
 
 @onready var camera: Camera3D = $Camera3D
 
+signal selected_changed
+signal inventory_requested(container: Node)   ## the HUD opens the inventory screen
+
+const HOTBAR := 8
+var inventory := Inventory.new(40)   ## slots 0-7 are the hotbar
+var selected := 0
+var ui_open := false                 ## set by the inventory screen; freezes look/move
+var hands: Node3D
+
 var _gravity: float = ProjectSettings.get_setting("physics/3d/default_gravity", 9.8)
 
 func _ready() -> void:
-	Input.mouse_mode = Input.MOUSE_MODE_CAPTURED
+	if not get_parent().get("menu_mode"):   # title-screen backdrop: leave the cursor alone
+		Input.mouse_mode = Input.MOUSE_MODE_CAPTURED
+	hands = preload("res://scripts/player/hands.gd").new()
+	hands.name = "Hands"
+	hands.player = self
+	camera.add_child(hands)
+
+func held_item_id() -> String:
+	var s = inventory.get_slot(selected)
+	return s.id if s != null else ""
+
+func select(i: int) -> void:
+	selected = wrapi(i, 0, HOTBAR)
+	selected_changed.emit()
+
+## Called by workbenches and chests (container = null for plain crafting).
+func open_inventory(container: Node) -> void:
+	inventory_requested.emit(container)
 
 func _unhandled_input(event: InputEvent) -> void:
+	if ui_open:
+		return
 	if event is InputEventMouseMotion and Input.mouse_mode == Input.MOUSE_MODE_CAPTURED:
 		rotate_y(-event.relative.x * mouse_sensitivity)
 		camera.rotate_x(-event.relative.y * mouse_sensitivity)
 		camera.rotation.x = clamp(camera.rotation.x, -1.4, 1.4)
-	elif event is InputEventKey and event.pressed and event.keycode == KEY_ESCAPE:
-		Input.mouse_mode = Input.MOUSE_MODE_VISIBLE
 	elif event is InputEventMouseButton and event.pressed:
+		# Recapture after the window lost focus (ESC now opens the pause menu).
 		Input.mouse_mode = Input.MOUSE_MODE_CAPTURED
 
 func _physics_process(delta: float) -> void:
@@ -43,13 +70,14 @@ func _physics_process(delta: float) -> void:
 	elif not is_on_floor():
 		velocity.y -= _gravity * delta
 
-	if Input.is_action_just_pressed("jump") and is_on_floor():
+	var jump := Input.is_action_just_pressed("jump") and not ui_open
+	if jump and is_on_floor():
 		velocity.y = jump_velocity
-	elif Input.is_action_just_pressed("jump") and global_position.y > float_y - 0.4 and global_position.y < float_y + 0.3:
+	elif jump and global_position.y > float_y - 0.4 and global_position.y < float_y + 0.3:
 		# Swim-jump: kick up out of the water to climb onto a bank.
 		velocity.y = jump_velocity
 
-	var input_dir := Input.get_vector("move_left", "move_right", "move_forward", "move_back")
+	var input_dir := Vector2.ZERO if ui_open else Input.get_vector("move_left", "move_right", "move_forward", "move_back")
 	var direction := (transform.basis * Vector3(input_dir.x, 0, input_dir.y)).normalized()
 
 	var current_speed := sprint_speed if Input.is_key_pressed(KEY_SHIFT) else speed