feat(io): bundle background music as classpath resources

Switch OggMusicBackend from streaming an external, git-ignored music/ folder
(relative to the run directory) to streaming bundled classpath resources under
/music/, so the four manor tracks ship inside the JAR and play out of the box —
consistent with how fonts are bundled. The play loop reopens the resource each
iteration via a BufferedInputStream (the vorbisspi SPI needs mark/reset to read
the OGG header). Missing tracks still fall back to silence.

Add the four converted OGG Vorbis tracks (manor-theme/cellar-drone/musicbox/
lament) under src/main/resources/music/, un-ignore that path, and re-anchor the
gitignore so only the top-level MP3 source folder stays ignored. Update
docs/music.md for the bundled approach and conversion recipes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Semesterprojekt/.gitignore

@@ -44,5 +44,7 @@ build/
 # Local save games and settings
 saves/
 
-# External background-music files (not bundled)
-music/
+# Source audio (MP3 originals); only the converted .ogg under
+# src/main/resources/music/ is tracked and bundled. Anchored so it
+# ignores the top-level source folder but not the resources path.
+/music/