fix(m3u): wait for the playlist entity before hydrating hidden groups

The startup inventory publishes playlists only after settings and EPG
reconciliation, while a restored or deep-linked playlist route waits for
the metadata read alone. The route session's full read could therefore
finish first, and updateOne dropped the hydration for the missing entity
for the whole session. The hydration now waits for the entity.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/architecture/m3u-playlist-module.md

@@ -638,6 +638,10 @@ channel-list-container/
   restart. `M3uWorkspaceRouteSession` copies it from the full playlist read into
   the store (`updatePlaylistMeta` with `persist: false`) before publishing the
   channels, unless the store value changed while that read was queued (#1894).
+  A restored or deep-linked route can finish that read before the startup
+  inventory, which waits for settings and EPG reconciliation, publishes the
+  playlist; the hydration then waits for the entity, since `updateOne` drops an
+  update for a missing one.
 - The groups route keeps the manage action reachable even when every group is
   hidden by separating "playlist has no groups" from "no visible/search-matching
   groups" empty states.