feat(music): W22 shared listening + release 0.6.0

Personal playlist on a dedicated music player (browse/play/prev/next/
seek/volume/reorder/remove, .pls/.m3u import), plus per-person shared
listening: broadcast your track over presence, peers tune in and stream
it point-to-point over the files plane. Playback is timeline-synced
(play/pause/skip/seek mirror with no drift) with gapless prefetch of the
next track and independent per-source volume per listener.

In the 3-column layout the playlist gets its own card stacked under the
chat, with a resizable divider and its own scrollbar; other layouts keep
it in the Controls panel.

Breaking wire change: gossip protocol v5 (presence gains music fields),
so 0.6.0 peers cannot share a swarm with 0.5.x. Version bumped 0.5.1 ->
0.6.0; CHANGELOG updated.

Untrusted-input handling: broadcast track name sanitized and size
cap-checked at gossip ingest, fetched bytes confirmed audio before
decode, only the descriptor rides gossip (bytes go point-to-point, one
fetch in flight).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/gossip_rebootstrap.rs

@@ -64,6 +64,7 @@ fn state(name: &str, addr: EndpointAddr) -> PeerState {
         sharing: None,
         avatar: Default::default(),
         game: None,
+        music: None,
     }
 }