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.

MainMenu.gd.uid

@@ -0,0 +1 @@
+uid://p7yl0hc43yai