Follow-up on the salvaged #71610 commits:
- acquire/release a scoped lock on relay_url+pubkey in connect/disconnect
(IRC pattern) so two profiles can't drive one Buzz identity — duplicate
replies and split de-dupe state; +2 tests
- negative-cache _resolve_user_name failures so a profile-less pubkey
doesn't re-hit 'users get' every poll sweep (flagged by @jethac on the PR)
- register user-guide/messaging/buzz in website/sidebars.ts (page was
unreachable — the #63359 trap)
