A boost or a favourite given again is sent again

Every boost of a post was announce-{post}, and every favourite like-{post}:
after an Undo, giving it again made the same activity id, which the
delivery queue had already delivered and dropped (and a server that
remembers ids would ignore). Found by decePub's e2e actions against the
pasture, which unboost and boost again. Now each boost and favourite is an
activity of its own, as Mastodon's are, and an Undo names the one it ends;
a concurrent second boost is removed after saving. An edit's Update is
named to the millisecond, a quote approval's to the tick.

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

FEDERATION.md

@@ -227,7 +227,8 @@ Posts with a location (shown to nearby users of this server) never leave the ser
   carry is deleted (then 410); a persona's boost id sends a browser to the boosted post. `Follow`, `Like`, `Block` and
   the `Accept`, `Reject` and `Undo` that answer them do not, because serving them would reveal who follows, likes and
   blocks whom. Neither do `Update`, `Delete`, `EmojiReact`, `QuoteRequest` and its answers, `Flag`, `Ignore` or poll
-  votes. All of them are always sent with their object embedded.
+  votes. All of them are always sent with their object embedded. Every boost and every favourite is an activity of its
+  own (`announce-{boost}`, `like-{favourite}`), so one given again after its `Undo` is new to the server it reaches.
 - **Hashtags.** A post's `Hashtag` links go to `/tags/{tag}`, a public page of this server's public posts with that tag.
 - **Delivery.** Failed deliveries are retried with Mastodon's backoff (16 attempts). A host that keeps failing is paused,
   starting at an hour and growing to a week. A server can also take a Follow (202) and drop it afterwards, as Pleroma