#2697 gatorwalk-factory swamp-club adapter: assign robustness (stale assignees, dropped entries, default username)

Findings from verify-reviews on the swamp-club Lab adapter (swamp-club #2631, gatorwalk-factory/extensions/models/_lib/swamp_club.ts and swamp_club.ts). None blocked the gate; they were left for a follow-up.

1. (medium) assign PATCHes the union of the issue's current assignee ids and the new user. swamp-club rejects the whole list with 400 One or more assignee IDs are not eligible when any id is no longer on the team, so an issue carrying a stale assignee makes every later assign fail as invalid, and the message points at the PATCH rather than the stale assignee. Fix: filter kept assignees against the eligible list already fetched, and say which were dropped, or name the ineligible assignee in the error.
2. (low) getIssue skips assignee entries without a string userId, and assign then PATCHes without them, which would unassign them. It should fail as upstream instead, as getIssue does for other malformed replies.
3. (low) assign's default username comes from the stored login even when the resolved server is a different one. Use it only when the url matches the stored login's serverUrl; otherwise require username.
4. (low) Read-then-write assignee union can lose a concurrent edit (already listed under known gaps in DESIGN.md).
5. (low) The stored-login server match is a plain string comparison after trimming trailing slashes; compare parsed origins instead.
6. (low) An auth.json with a non-string serverUrl throws a raw TypeError; the legacy-domain rewrite matches only the exact string https://swamp.club.