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

CLAUDE.md

@@ -562,6 +562,10 @@ tools/pasture/run.sh down                                            # removes e
 - **Threads (`scenarios/threads.sh`, needs mastodon):** mastouser follows alice, carol answers alice's public post,
   and opening the thread on Mastodon finds carol's reply through alice's `replies` (the scenario makes Mastodon's copy
   look ten minutes old: Mastodon reads a thread's replies only five minutes after it first holds the post). 5 checks.
+- **Followers synchronisation (`scenarios/followsync.sh`, needs mastodon):** PrivaPub forgets mastouser's follow of
+  alice (mongo), alice's followers-only post naming mastouser makes Mastodon read her roll-call and drop it; then
+  Mastodon forgets a follow (rails) and alice's next followers-only post makes it send the `Undo` PrivaPub applies.
+  Follows are checked in Mastodon's database: its API caches relationships for a day. 8 checks.
 - **Pins (`scenarios/pins.sh`, needs mastodon):** a Mastodon account pins and unpins while alice follows it, alice pins
   and unpins while it follows her, a fresh account's earlier pin shows once PrivaPub resolves it, and following it brings
   its earlier posts (its outbox). 9 checks.