Add a main menu scene (#47)

MainMenu.tscn is now project.godot's entry point: Play / Map Editor
(disabled, explains itself - #48 wires it up) / Quit (hidden in web
builds), styled like the rest of the game's menus and reusing the
starfield shader for its background.

Play calls GameState.reset_new_game(), so a second playthrough in the
same process starts fresh instead of inheriting whatever the previous
run left in the autoload - existing tests still load Main.tscn
directly and are unaffected.

Part of the #40 conquest-mechanic breakdown.

tests/test_main_menu.gd

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+extends SceneTree
+
+# Headless gameplay test: run with
+#   godot --headless -s res://tests/test_main_menu.gd
+# Verifies:
+#   1. the menu loads with Play/Map Editor/Quit, the editor button starts
+#      disabled and explains itself instead of silently doing nothing
+#   2. Quit's visibility follows OS.has_feature("web") (this platform is
+#      not a web export, so this run exercises the "visible" branch; the
+#      condition itself is what's under test, not the concrete boolean)
+#   3. Play resets GameState to a fresh game AND reaches Main.tscn
+
+var menu: Node
+var game_state: Node
+var elapsed: float = 0.0
+var stage: int = 0
+
+
+func _initialize() -> void:
+	game_state = root.get_node("GameState")
+	menu = load("res://MainMenu.tscn").instantiate()
+	root.add_child(menu)
+	current_scene = menu
+
+
+func _process(delta: float) -> bool:
+	elapsed += delta
+	match stage:
+		0:
+			# A scene added to the tree inside _initialize() has not had its
+			# _ready() flushed yet - see test_planet_setups.gd for the same
+			# hazard - so the menu's @onready buttons aren't valid until a
+			# frame has actually processed.
+			if elapsed < 0.1:
+				return false
+			if menu.editor_button.enabled:
+				return _fail("map editor button is not disabled ahead of #48")
+			if menu.quit_button.visible != (not OS.has_feature("web")):
+				return _fail("quit button visibility did not follow OS.has_feature('web')")
+
+			_press(menu.editor_button)
+			elapsed = 0.0
+			stage = 1
+			return false
+		1:
+			if elapsed < 0.1:
+				return false
+			if "coming soon" not in menu.status_label.text.to_lower():
+				return _fail("disabled editor button press did not explain itself")
+			if current_scene != menu:
+				return _fail("pressing the disabled editor button changed scenes")
+
+			# Dirty the game state exactly like an in-progress run would,
+			# so a genuine reset is distinguishable from a coincidental match.
+			game_state.bank_inventory.set_amount(GameState.RESOURCE_ORE, 999)
+			game_state.add_building(GameState.BUILDING_MOTHERSHIP, Vector2.ZERO)
+			game_state.surviving_ship_count = 1
+			game_state.planet_capture["chunk:1:1"] = {"captured": [true], "conquered": true}
+
+			_press(menu.play_button)
+			elapsed = 0.0
+			stage = 2
+			return false
+		2:
+			if elapsed < 0.3:
+				return false
+			var on_main: bool = current_scene != null and current_scene.name == "Main"
+			var reset_ok: bool = game_state.rocks == 500 and game_state.buildings.is_empty() \
+				and game_state.surviving_ship_count == GameState.BASE_FLEET_SIZE \
+				and game_state.planet_capture.is_empty()
+			print("on_main=%s reset_ok=%s rocks=%d buildings=%d ship_count=%d" % [
+				on_main, reset_ok, game_state.rocks, game_state.buildings.size(),
+				game_state.surviving_ship_count
+			])
+			if not on_main or not reset_ok:
+				return _fail("Play did not reset to a fresh game before reaching Main.tscn")
+			print("TEST_MAIN_MENU: PASS")
+			quit(0)
+			return true
+	return false
+
+
+func _press(button: IconButton) -> void:
+	var down: InputEventScreenTouch = InputEventScreenTouch.new()
+	down.pressed = true
+	button._gui_input(down)
+	var up: InputEventScreenTouch = InputEventScreenTouch.new()
+	up.pressed = false
+	button._gui_input(up)
+
+
+func _fail(message: String) -> bool:
+	print("TEST_MAIN_MENU: FAIL (%s)" % message)
+	quit(1)
+	return true

tests/test_main_menu.gd.uid

@@ -0,0 +1 @@
+uid://dgf2ichdb7d4j