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>

src/network/gossip.rs

@@ -645,6 +645,29 @@ impl RoomState for IrohGossipState {
                                             let cleaned = crate::sanitize::sanitize_game_label(&g);
                                             (!cleaned.is_empty()).then_some(cleaned)
                                         });
+                                        // Music presence is untrusted peer data:
+                                        // the track name is display text (sanitize
+                                        // + cap like the game label) and the size
+                                        // bounds a future fetch (reject anything
+                                        // outside the attachment cap).
+                                        state.music = state.music.and_then(|mut m| {
+                                            let name = crate::sanitize::sanitize_game_label(&m.name);
+                                            if name.is_empty() || !crate::files::size_within_cap(m.size) {
+                                                return None;
+                                            }
+                                            m.name = name;
+                                            if m.next_id.is_some() {
+                                                let ok = m
+                                                    .next_size
+                                                    .map(crate::files::size_within_cap)
+                                                    .unwrap_or(false);
+                                                if !ok {
+                                                    m.next_id = None;
+                                                    m.next_size = None;
+                                                }
+                                            }
+                                            Some(m)
+                                        });
                                         // Bound an insider's advertised address set
                                         // before we retain it / hand it to the dialer
                                         // (Tier C F-01).
@@ -959,6 +982,7 @@ mod tests {
             sharing: None,
             avatar: crate::avatar::Avatar::default(),
             game: None,
+            music: None,
         }
     }