P5 done: a key we cannot fetch for now gets 503, and follow/like/block ids stay private on purpose

- When a sender's key cannot be fetched because its server timed out or answered 5xx, the inbox answers 503 with
  Retry-After: 300 instead of 401, so Mastodon 4.7 retries rather than switching to RFC 9421 signatures we do not
  verify yet. The fetcher's failure cache now remembers whether a failure was temporary.
- Follow, Like, Block, Accept, Reject and Undo ids are deliberately not dereferenceable: serving them would publish
  who follows, likes and blocks whom. They are always sent with their object embedded.

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

docs/ROADMAP.md

@@ -10,7 +10,7 @@ Written 2026-10-01 from the original 2023 code, the decePubClient UI, a federati
 - [x] P2 Mastodon client API: v1.4.0, deployed 2026-10-01; OAuth and the anonymous API verified on production, the signed-in API verified locally (no real-client login on production yet)
 - [x] P3 Social features: v1.5.0, deployed 2026-10-01; media, proxy, blocks, mutes, bookmarks, pins and reports verified by tests and locally (no upload on production yet)
 - [x] P4 Groups and privacy features: v1.6.0, deployed 2026-10-01; communities, circles and local-only located posts verified by tests. v1.6.1 adds the pasture (`tools/pasture/`): live interop with GoToSocial 0.22.1 passes all 25 checks, three runs in a row. Lemmy and a live Mastodon circle member are not run yet; the pasture has GoToSocial only
-- [ ] P5 Lose nothing: wire tolerance, full objects, raw capture (see `docs/INTEROP.md`)
+- [x] P5 Lose nothing: v1.7.0 to v1.9.1, deployed 2026-10-01; parsing, typed details, provenance, downvotes, tombstones, federated blocks, all checked live against GoToSocial. Book reviews and forum threads keep their raw form only (typed in P7 and P8)
 - [ ] P6 Emoji, polls, quotes, reactions, cards, players
 - [ ] P7 Threads, communities, moderation, the social graph
 - [ ] P8 Signatures, discovery, the long tail
@@ -414,7 +414,7 @@ it, raw where it doesn't.
   - Persona usernames match Mastodon's and Misskey's pattern (groups already did).
   - Every term we emit is defined in our JSON-LD context.
   - `Vary: Accept`.
-  - Still open: every activity id dereferences.
+  - Not done on purpose: see "Deliberately not done" below.
 - **Owner decisions that were small** (done in v1.7.0): blocks federate (`Block`, `Undo{Block}`); a persona's and a
   group's `published` is a random day up to two weeks before its creation (migration `_007`).
 - **Parsing every shape** (done in v1.8.0, `Federation/Objects/ObjectShapes.cs`):
@@ -441,7 +441,11 @@ it, raw where it doesn't.
   - A deleted object's id is kept for 90 days (W9), and its `ObjectRecord` is removed with the post.
 - **Typed details reach clients** (done in v1.9.0) as `Status.privapub`.
 - **Delivery:** a 503 with `Retry-After` is waited out like a 429 and no longer counts against the host (done in v1.9.0).
-  Still open: answering 503 ourselves when a key fetch fails temporarily, and making every activity id dereference.
+  We answer 503 with `Retry-After` ourselves when a sender's key cannot be fetched for a temporary reason (v1.9.1),
+  so Mastodon 4.7 retries instead of switching to RFC 9421.
+- **Deliberately not done: dereferenceable `Follow`, `Like`, `Block`, `Accept`, `Reject` and `Undo` ids.** Serving them
+  would publish who follows, likes and blocks whom, which our collections deliberately hide. Every one of them is sent
+  with its object embedded, which is all Misskey needs. `Create` and `Announce` ids do dereference.
 
 #### P6 What people see: emoji, polls, quotes, reactions, cards, players
 - **Custom emoji** on posts, names, fields and poll options, proxied.