feat(paket b): Umbenennen erreicht alle — PATCH /title und RENAMED im Feed (D85)

Backend: PATCH /documents/{id}/title mit expectedVersion (409 bei
veralteter Version, 400 bei leerem/zu langem Titel), neuer ChangeType
RENAMED (strukturell, immer Meilenstein), der Feed stellt den neuen Titel
im Klartext zu (ChangeEvent.title). Unter derselben Stripe-Sperre wie die
Inhalts-Patches; Owner-Vormerkung in der API-Beschreibung. Vier neue
Cucumber-Szenarien, zwei Unit-Tests.

Frontend: Der Zeilen-Stift eines Server-Dokuments benennt über den Server
um (optimistisch, 409-Retry, Rücknahme + Warnung bei Fehlschlag); fremde
Umbenennungen kommen als RENAMED über den Feed in Chip und Menü.
URL-Dokumente verlieren den Stift — ihr Name ist die URL.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/examples/werkbaum.werkbaum

@@ -154,6 +154,7 @@
       - [^] #col.live.upload: Share: put a local plan on the server (S)  %% a button in the editor title bar, see D81
       - [?] #col.live.cursors: Cursors and who else is here (S)
       - [ ] #col.live.owner: An owner password guards managing a shared plan (M)  %% returned once on creation, admin can reclaim
+      - [x] #col.live.rename: Renaming reaches everyone (S)  %% PATCH /title + RENAMED in the feed, see D85
       - [-] #col.live.ws: Transport over a websocket (S)  %% plain HTTP turned out to be enough
       - [-] #col.live.own: A merge algorithm of our own (XL)  %% solved problem, do not reinvent
   - [?] #mmd: Mermaid plugin (XL)
@@ -918,6 +919,11 @@
   another browser. Management actions bind to it; an admin with the master
   password can still reclaim a document.
 
+#col.live.rename
+  The title is server metadata, so renaming patches it with a version check
+  and the change feed hands the new title to every client in plain text. URL
+  documents lost their pencil instead - their name is the URL.
+
 #col.live.cursors
   Seeing where the others are. A by-product of the merge layer rather than a
   feature in its own right.