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

docs/ROADMAP.md

@@ -672,7 +672,8 @@ it, raw where it doesn't.
   - re-run WebFinger on a rename;
   - inbound `Block`; `Add`/`Remove` of pins: **done 2026-10-05** (both ways, a community's pins too, the `featured`
     collection read with an account's counts; checked live against Mastodon);
-  - FEP-8fcf followers sync;
+  - FEP-8fcf followers sync: sending **done 2026-10-06** (owner decision; the digest of a persona's followers on the
+    receiving server, and a signed roll-call listing only them; Mastodon mends both ways from it, checked live);
   - `indexable`/`discoverable`/`searchableBy`;
   - edit history from `formerRepresentations`;
   - PeerTube reply rules and `ApproveReply`.