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

docs/INTEROP.md

@@ -144,7 +144,7 @@ Priorities, used throughout:
 | Link attachments as the card source | P1 | `Status.card` |
 | Inbound `Move` with Mastodon's checks (`target` re-fetched, its `alsoKnownAs` lists the old account, 7-day lock); move each persona's follow | P1 | `Account.moved` |
 | Re-run WebFinger when `preferredUsername` changes; key accounts on the actor id | P2 | `Account.acct` |
-| Inbound `Block`: stop delivering, hide | P2 | `relationship.blocked_by` |
+| Inbound `Block`: stop delivering, hide (**done** 2026-10-04) | P2 | `relationship.blocked_by` |
 | `Add`/`Remove` featured (pins, tags) | P2 | `GET /accounts/:id/statuses?pinned=true` |
 | Remote like and boost totals | P2 | counts |
 | Publish `context` and a paged `replies` | P2 | — |
@@ -182,7 +182,8 @@ Findings:
     makes Mastodon delete its copy. Reporting the numeric-inbox recipient loss upstream is still worth doing.
 - **With SecureMode on** (all six peers, 2026-10-04) every federation check passes: Mastodon, GoToSocial, Misskey,
   Sharkey, Akkoma and Lemmy all sign their fetches, and fetch our instance actor's key unsigned first.
-- Inbound `Block` from Mastodon is not enforced yet (P7).
+- Inbound `Block` from Mastodon is enforced since 2026-10-04 (`BlockHandler`, `BlockedBy`): follows end both ways, the
+  blocker is hidden from the persona, and the relationship says `blocked_by`.
 
 ### GoToSocial: 0.22.1 (2026-07-20)