#148: keep the theme's master in the repository

SPACE_4.wav is the source the shipped theme is made from, so it belongs
under version control - but not in the game.

It lives in audio/masters/ with an empty .gdignore beside it, which makes
Godot skip the folder outright: nothing in it is imported into res://, and
nothing in it is carried into an export. Left where it was, in the project
root, its 59MB would be imported and then packed into every Android and web
build for a file nothing opens - the game loads audio/space_4.mp3, the same
5m36s at 6.7MB.

Its .import file goes with it, since there is no longer anything to import,
and the test runner stops copying the folder for the same reason it already
skips android/ and builds/: 59MB copied on every single run, for a folder no
test can even see.

README.md next to it records which shipped file each master became and the
command that made it, so the pair cannot drift apart silently.

tests/run_tests.sh

@@ -173,9 +173,12 @@ if [[ "$IN_PLACE" != "1" ]]; then
 	# keep being run.
 	find "$run_dir" -mindepth 1 -maxdepth 1 ! -name '.godot' -exec rm -rf {} +
 	# android/ and builds/ are export artefacts no test touches, and the .apk
-	# is 30MB of nothing useful here.
+	# is 30MB of nothing useful here. audio/masters/ is the same argument:
+	# Godot skips it (.gdignore) so no test can reach it, and it is 59MB of
+	# source WAV that would be copied on every single run.
 	tar -C "$PROJECT_DIR" \
 		--exclude=./.godot --exclude=./android --exclude=./builds --exclude='*.apk' \
+		--exclude=./audio/masters \
 		-cf - . 2>/dev/null | tar -C "$run_dir" -xf -
 fi