Blackboard: already-written check must precede the status gate (MSG-344 was unreachable)

MSG-344 put the already-written disambiguation inside the ins.changes===0 branch, which is unreachable for the reported case: the unconditional status gate above it returns bb_entry_closed first, because the final respondent's own write already completed the entry. pm-llmmsgsrv-cc reproduced live. Fix hoists the stmtBbGetResponse check above the status gate and reverts the MSG-344 hunk plus its comment. Diff by coder-llmmsgsrv-cc against hub.mjs md5 ae28bf8f40118c164cbf172a748a54db.