A sender's digest of its followers here is compared and mended

FEP-8fcf, received. When a delivery's Collection-Synchronization header digests the sender's followers on PrivaPub
otherwise than the personas following it, a job reads the list the header names (on the sender's origin, signed by the
instance actor): a follow the list leaves out ends, only when the list is the one the digest describes; a request it
lists is taken as accepted; a persona it lists that follows nothing there sends Undo{Follow}, as Mastodon does. Each
claiming delivery is compared once.

Checked live (scenarios/followsync.sh, now 15 checks): PrivaPub ends a follow Mastodon lost and undoes one only
Mastodon remembered.

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

FEDERATION.md

@@ -72,7 +72,7 @@ covered by unit tests written in their documents' shape.
   `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")
+  (sent and honoured; 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-8967 (link
@@ -179,6 +179,10 @@ persona's posts passed on to its followers. A deleted post answers 410 with a `T
   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.
+  Honoured the other way: when a delivery's header digests the sender's followers here otherwise than the personas
+  following it, PrivaPub reads the list it names (same origin, signed by the instance actor). A follow the list leaves out
+  ends, but only when the list is the one the digest describes; a request it lists is taken as accepted; a persona it
+  lists that follows nothing there sends `Undo{Follow}`, as Mastodon does.
 - **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