Interop research: every major platform's wire shapes, gotchas, and what PrivaPub still drops

docs/INTEROP.md collects five research passes from 2026-10-01: Mastodon 4.7 and GoToSocial 0.22, the Misskey and
Pleroma families, the threadiverse (Lemmy 0.19/1.0, PieFed, Mbin, NodeBB), media and long-form (PeerTube, Loops,
Pixelfed, WordPress, Ghost, events, audio, books, Threads, Flipboard, Bridgy Fed), and cross-cutting FEPs and
signatures. Each claim was checked against source code or live fetches, with dates and versions.

It is checked against our own code: what is already right, three cheap things that are wrong today (summary read as a
CW on every type, unchecked usernames, an undefined context term), what the rich client needs kept (a post kind with
typed payloads, raw capture and provenance for the details view), and the six privacy choices the owner has to make.

The roadmap's P5 becomes P5 to P8, ordered by that evidence.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

FEDERATION.md

@@ -29,9 +29,12 @@ PrivaPub is an ActivityPub server written in C#. This document follows
 - [FEP-67ff: FEDERATION.md](https://codeberg.org/fediverse/fep/src/branch/main/fep/67ff/fep-67ff.md)
 - [FEP-f1d5: NodeInfo in Fediverse Software](https://codeberg.org/fediverse/fep/src/branch/main/fep/f1d5/fep-f1d5.md)
 - [FEP-2c59: Discovery of a WebFinger address from an ActivityPub actor](https://codeberg.org/fediverse/fep/src/branch/main/fep/2c59/fep-2c59.md)
+- [FEP-1b12: Group federation](https://codeberg.org/fediverse/fep/src/branch/main/fep/1b12/fep-1b12.md) (communities; see "Groups")
 
-Planned: FEP-1b12 (communities), FEP-8fcf (followers synchronisation), FEP-5feb (`indexable`), FEP-7628 (Move),
-FEP-044f (quotes).
+Planned (see `docs/ROADMAP.md`, phases P5 to P8, and the per-platform notes in `docs/INTEROP.md`): FEP-044f (quotes),
+FEP-9967 (polls), FEP-c0e0 (emoji reactions), FEP-9098 (custom emoji), FEP-7888 and FEP-f228 (threads), FEP-7628 (Move),
+FEP-8fcf (followers synchronisation), FEP-5feb (`indexable`), FEP-8967 (link attachments), FEP-521a and FEP-8b32 (keys
+and integrity proofs), FEP-ae0c (relays).
 
 ## Actors