fix(explore): list every library copy of a title, not one per server

`MediaServerClient.findByExternalIds` returned `MediaItem?`, so the Explore
"In these libraries" chooser could never show more than one copy per server.
A movie held by both a 4K library and an HD library on one Plex server
therefore resolved to whichever copy came back first, with no way to reach
the other.

Return every id-verified match instead. `/library/all` is already
server-wide and each `Metadata` entry carries its own `librarySectionID`,
so both copies come back labelled with no extra request; Plex was simply
taking `Metadata[0]` and the title ladder was returning on its first hit.
An exact-guid hit no longer short-circuits the title search either — a
library still on a legacy agent has a different primary guid and is
invisible to the `guid=` filter.

Copies are deduped by global key and ordered best-first, and each row now
states its resolution, since library names need not mention it.

Resolution passes merge rather than replace: the cross-server fan-out logs
and skips per-server failures, so a later pass can come back short a server
that answered an earlier one, and a failed pass no longer claims the title
left the library. Duplicate keys fold field by field, because Jellyfin's
library stamp is a best-effort ancestors lookup that returns the item bare
when it fails and an unstamped row is indistinguishable from its sibling.
Focus nodes are keyed by copy and reclaimed after a merge re-sorts the
rows, so a dpad user is not thrown to a different copy.

close #1754

lib/media/media_server_client.dart

@@ -504,7 +504,7 @@ abstract class MediaServerClient {
   /// has no external mapping for the item.
   Future<ExternalIds> fetchExternalIds(String itemId);
 
-  /// Reverse lookup: find a library movie/show matching any of [ids].
+  /// Reverse lookup: find every library movie/show matching any of [ids].
   ///
   /// Neither backend can filter by external id — Plex's `guid=` matches only
   /// the primary `plex://` guid (verified on PMS 1.43) and Jellyfin dropped
@@ -512,7 +512,16 @@ abstract class MediaServerClient {
   /// title and verify candidates against their exact external ids. Title
   /// alone never produces a match.
   ///
-  /// [titles] are tried in order until one yields an id-verified candidate;
+  /// One title can own several library items: a server with a 4K section and
+  /// an HD section holds two rating keys for the same movie, and a library
+  /// still on a legacy agent carries a different primary guid than its modern
+  /// sibling. Implementations MUST return every id-verified copy rather than
+  /// the first, and MUST NOT truncate — external-id verification is the only
+  /// bound, so a long list means the user genuinely owns that many copies and
+  /// the caller (the Explore "In these libraries" chooser) exists to show
+  /// them. Ordering is the implementation's, and callers re-sort.
+  ///
+  /// [titles] are tried in order until one yields id-verified candidates;
   /// pass the entry's own title first and broader forms after (see
   /// `titleMatchCandidates`). A sequel entry's own title never matches its
   /// parent show, which is why more than one is needed. [year] applies a ±1
@@ -520,22 +529,21 @@ abstract class MediaServerClient {
   /// season's, not the show's.
   ///
   /// [plexGuid] is a Plex-only escape hatch: a `plex://show/…` guid the caller
-  /// already holds, which the local server *can* filter on exactly, skipping
-  /// the title search. It is never resolved over the network — only Plex
-  /// Discover catalog items carry one, in their own rating key. Other backends
-  /// ignore it.
+  /// already holds, which the local server *can* filter on exactly. It is
+  /// never resolved over the network — only Plex Discover catalog items carry
+  /// one, in their own rating key. Other backends ignore it.
   ///
-  /// [season] gates the result: when the entry maps to season 2+ of a longer
-  /// series, a match is only returned if the server actually has that season.
+  /// [season] gates the results: when the entry maps to season 2+ of a longer
+  /// series, a candidate is only kept if the server actually has that season.
   /// Implementations MUST gate only on [ExternalSeasonRef.agreedSeason] — the
   /// provider a library numbers its seasons by is a server-side setting no
   /// dataset supplies, so a disagreeing ref is left ungated rather than gated
   /// on a guess.
   ///
-  /// Returns null when this server has no match or [kind] is not movie/show.
-  /// Used to match external catalog items (Explore tab) back to the user's
-  /// libraries.
-  Future<MediaItem?> findByExternalIds(
+  /// Returns an empty list when this server has no match or [kind] is not
+  /// movie/show. Used to match external catalog items (Explore tab) back to
+  /// the user's libraries.
+  Future<List<MediaItem>> findByExternalIds(
     ExternalIds ids, {
     required MediaKind kind,
     List<String> titles = const [],