A remote account's Block of a persona is enforced

Inbound Block was dropped as an unknown type (the pasture's last
Mastodon expected failure, G-0002). Now, as Mastodon does it: the
follows between the blocker and the persona end here too (nothing is
sent back; the blocker already ended its side), the blocker's posts and
notifications are hidden from the persona and kept out of its home, the
persona's posts are no longer addressed to the blocker by mention or
reply, and the relationship says blocked_by. Undo{Block} lifts it. The
block is kept in BlockedBy, unique per persona and blocker.

The Mastodon scenario checks blocked_by instead of expecting a failure.

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

FEDERATION.md

@@ -118,6 +118,7 @@ Received:
 | `Announce` | counted and notified for local posts; shown to followers of the announcer, with the original refetched from its origin |
 | `Delete` | deletes the object, or the actor and its follows; a deleted object id is remembered for 90 days, so a late `Create` cannot bring it back |
 | `Flag` | becomes a report for this server's moderators |
+| `Block` of a persona | the follows between them end, the blocker's posts and notifications are hidden from the persona, nothing of the persona's is addressed to the blocker, and the relationship says `blocked_by`; `Undo{Block}` lifts it |
 
 Sent: `Follow`, `Undo{Follow}`, `Create{Note}`, `Create{Question}` and poll votes, `EmojiReact` and its `Undo`, `Update{Note}`, `Update{Person}`, `Delete{Tombstone}`, `Accept{Follow}`,
 `Reject{Follow}`, `Like`, `Announce` and their `Undo`, `Flag`. A deleted post answers 410 with a `Tombstone`.