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.

GameState.gd

@@ -344,6 +344,33 @@ func reset_progression() -> void:
 	vessel_levels.clear()
 
 
+# Called from the main menu's Play button (#47): the autoload survives
+# change_scene_to_file(), so without this a second playthrough in the same
+# process would start from wherever the last one left off instead of a
+# fresh game.
+func reset_new_game() -> void:
+	has_return_position = false
+	surviving_ship_count = BASE_FLEET_SIZE
+	surviving_vessel_types = PackedStringArray()
+	return_world_position = Vector2.ZERO
+	return_camera_zoom = 1.0
+	planet_name = ""
+	planet_id = ""
+	planet_definition_id = "rocky"
+	explored_chunks.clear()
+	bank_inventory = INVENTORY.new({RESOURCE_ORE: 500})
+	fleet_inventory = INVENTORY.new()
+	buildings.clear()
+	next_building_id = 1
+	stored_vessels.clear()
+	vessel_deliveries.clear()
+	extractors.clear()
+	transporters.clear()
+	planet_capture.clear()
+	reset_progression()
+	reset_formation()
+
+
 # --- building stock and production queue (#16) -----------------------------
 
 # Since #12, a building cannot just deduct from the bank at the moment of