feat(io): OggMusicBackend + SwingIO wiring; music/ folder + docs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Semesterprojekt/docs/music.md

@@ -0,0 +1,16 @@
+# Background music (GUI)
+
+Background music plays only in the Swing GUI. It is **data-driven and external**:
+
+- Put `.ogg` (OGG Vorbis) files in a `music/` folder next to where you run the game.
+  This folder is git-ignored and never bundled in the JAR.
+- A room plays a track via an optional `music:` field in `rooms.yaml`, e.g.
+  `music: manor-theme.ogg`. Entering a room with a different track cross-fades to it;
+  a room with no `music:` field fades to silence; the same track keeps playing
+  seamlessly across rooms.
+- Volume is the **Music** setting (Off / Low / Medium / High) in the Settings menu,
+  persisted to `saves/settings.json`. Off disables playback.
+- Missing files are ignored (silent) — the game never crashes over absent audio.
+- The console version is always silent.
+
+Decoding uses the `vorbisspi` + `jorbis` libraries via the `javax.sound` SPI.