#148: the main theme plays, and keeps playing

"All the time" is the whole of the requirement and it decides the shape.
The theme has to survive the main menu becoming the game, the game becoming
a planet surface, and the surface becoming the game again - so the player
cannot live in any of those scenes, because change_scene_to_file() frees
whichever one is current.

It is parented to the tree root instead, beside the GameState autoload,
which is the same trick for the same reason. Music.ensure() is idempotent by
node name, so each scene asks for the music on the way up and gets the one
already playing rather than a second copy over the top of it. Two call
sites cover everything: MainMenu, and GameController - which is the root
script of BOTH the space scene and the planet surface.

Done this way rather than as an autoload deliberately: project.godot carries
the developer's own uncommitted window settings, and adding music should not
drag those into a commit.

It is also PROCESS_MODE_ALWAYS. Since #128 a dialog stops the tree, and a
soundtrack that cuts out every time someone reads a line is worse than no
soundtrack.

The theme ships as MP3 rather than the 59MB WAV that was sitting in the
project root. Same 5m36s, 6.7MB, encoded with LAME at 160k - 89% smaller,
which is the difference between a repository that carries its music and one
that carries a WAV forever in its history. The original file is untouched
and still there, untracked, so nothing is lost.

The test checks CONTINUITY rather than "a sound started": the same player,
never a second one, through two scene changes and a dialog.

controllers/GameController.gd

@@ -102,6 +102,10 @@ var keyboard_steering: bool = false
 
 
 func _ready() -> void:
+	# #148: the main theme, which outlives this scene - see Music.ensure().
+	# This script is the root of BOTH the space scene and the planet
+	# surface, so one call here covers the whole game.
+	Music.ensure(get_tree())
 	if joystick != null:
 		joystick.direction_changed.connect(_on_joystick_direction)