Public threads name their replies and context

Owner decision of 2026-10-06: a persona's public or unlisted post outside any group names its replies
(…/scribbles/{id}/replies) and its conversation's context (FEP-7888), the root's …/context that a reply inherits from
its parent, ours or another server's. Both list only the public and unlisted posts PrivaPub holds; followers-only,
circle, direct and local-only posts name neither and their collections answer 404. Checked live: opening alice's
thread on Mastodon finds carol's reply, which nobody there follows (scenarios/threads.sh).

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

FEDERATION.md

@@ -166,6 +166,11 @@ persona's posts passed on to its followers. A deleted post answers 410 with a `T
 
 - **Attachments** are `Document`s with `mediaType`, `name` (alt text), `blurhash`, `focalPoint`, `width` and `height`.
   Uploaded files have all metadata removed.
+- **Threads** (owner decision 2026-10-06). A persona's public or unlisted post outside any group names its `replies`
+  (`…/scribbles/{id}/replies`) and its conversation, `context` (FEP-7888): the root post's `…/scribbles/{id}/context`,
+  which a reply inherits from its parent, ours or another server's. Both list only the public and unlisted posts
+  PrivaPub holds, the context every one under the root (at most 500); followers-only, circle, direct and local-only posts
+  name neither, and their collections answer 404. Mastodon completes a thread from them.
 - **Pinned posts** are the actor's `featured` collection (`/trophies`); `featuredTags` is `/tattoos`. A pin or an unpin
   is told to the post's audience as `Add` or `Remove` on `featured`, as Mastodon tells it.
 - **Blocks are sent.** A blocked remote account receives `Block` from the blocking account (and `Reject{Follow}` if it