Restructure step 4: local library and media identity

- L1 (core, offline): release names with kinds (episode, movie, extra:
  OP/ED/Opening/Ending/NC/PV/OVA…), folder hints (season and name from
  parentheticals, ranges), one show across folders, gaps and next with
  skipped numbers, junk ignored, stable keys.
- L2 (app, device-owned cache): AniList match with synonyms and
  sequel walking, episode mapping across cours (AnimeFlow runs when
  signed in), art and episode titles, "Wrong show?" pins per show or
  folder, episode offsets; lazy, spaced requests, works offline.
- Files tab as a library: poster cards, show pages with banner, gap
  prompt, extras, movies, Wrong show? and Episode numbers.
- media.ref produced for local files, Jellyfin (provider ids) and links,
  carried in swaps and Up next; receivers name media from it.
- Hosting relays each Jellyfin item through its own server's login.

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

docs/PROTOCOL.md

@@ -354,8 +354,10 @@ media.ref = {"kind":"episode","title":"Show","anilistId":1,"anilistEp":6,
   is (the server drops non-integer ids and cuts `title` to 200
   characters). `kind` `episode`, `movie` or `extra` (else no ref); every
   other field optional; `season` absent for absolute numbering. At most
-  2 KiB of JSON, else `err` `bad`. **Reserved**: nothing produces it yet
-  (step 4).
+  2 KiB of JSON, else `err` `bad`. Apps send it with swaps and Up next
+  adds of files (the sender's library match), Jellyfin items (their
+  provider ids) and links (kind and title only); receivers name the
+  media by it (ARCHITECTURE.md "Library").
 
 ## Avatars