Deliveries to followers carry a digest of them, per server

Owner decision of 2026-10-06 (FEP-8fcf). A persona's delivery addressed to its followers carries a signed
Collection-Synchronization header naming its followers, its roll-call (…/groupies/roll-call) and the digest of its
accepted followers on the receiving server only. The roll-call answers a signed request with the persona's followers
on the signer's server and nobody else's.

Mastodon gives every Undo{Follow} it sends after reading a roll-call the same id (…#follows//undo), so a second one
looked like a copy: an Undo of a Follow that comes again while the follow it ends exists again is now kept once per
follow.

Checked live (scenarios/followsync.sh): Mastodon drops a follow PrivaPub lost, and undoes one it lost itself.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

FEDERATION.md

@@ -71,9 +71,11 @@ covered by unit tests written in their documents' shape.
 - [FEP-c0e0: Emoji reactions](https://codeberg.org/fediverse/fep/src/branch/main/fep/c0e0/fep-c0e0.md) (`EmojiReact`, and Misskey's
   `Like` with content)
 - [FEP-5feb: Search indexing consent](https://codeberg.org/fediverse/fep/src/branch/main/fep/5feb/fep-5feb.md) (`indexable`)
+- [FEP-8fcf: Followers collection synchronization across servers](https://codeberg.org/fediverse/fep/src/branch/main/fep/8fcf/fep-8fcf.md)
+  (sent; see "Followers synchronisation")
 
 Planned (see `docs/ROADMAP.md`, phases P7 and P8, and the per-platform notes in `docs/INTEROP.md`): FEP-9098 (custom
-emoji), FEP-7888 and FEP-f228 (threads), FEP-7628 (Move), FEP-8fcf (followers synchronisation), FEP-8967 (link
+emoji), FEP-7888 and FEP-f228 (threads), FEP-7628 (Move), FEP-8967 (link
 attachments), FEP-521a and FEP-8b32 (keys and integrity proofs), FEP-ae0c (relays).
 
 ## Actors
@@ -171,6 +173,12 @@ persona's posts passed on to its followers. A deleted post answers 410 with a `T
   which a reply inherits from its parent, ours or another server's. Both list only the public and unlisted posts
   PrivaPub holds, the context every one under the root (at most 500); followers-only, circle, direct and local-only posts
   name neither, and their collections answer 404. Mastodon completes a thread from them.
+- **Followers synchronisation** (FEP-8fcf, owner decision 2026-10-06). A persona's delivery addressed to its followers
+  carries a signed `Collection-Synchronization` header: its `followers`, its roll-call (`…/groupies/roll-call`) and the
+  digest (XOR of each id's SHA-256) of its accepted followers on the receiving server only. The roll-call answers a
+  signed request with the persona's followers on the signer's server and nobody else's; unsigned, 401. A server whose
+  view differs mends itself from it, Mastodon both ways (checked live). Mastodon's `Undo{Follow}` for a follow it never
+  knew of has one id per account (`…#follows//undo`); when it comes again after a new follow, it ends that follow too.
 - **Pinned posts** are the actor's `featured` collection (`/trophies`); `featuredTags` is `/tattoos`. A pin or an unpin
   is told to the post's audience as `Add` or `Remove` on `featured`, as Mastodon tells it.
 - **Blocks are sent.** A blocked remote account receives `Block` from the blocking account (and `Reject{Follow}` if it