Blackboard: multiple append-only writes per respondent

Allow a respondent to write MORE THAN ONCE on a blackboard entry, append-only: each write is a new timestamped row, never a revision, so a peer that already read an earlier answer still sees what it read and attribution holds. Today blackboard_write refuses a second write with bb_already_written; blackboard_amend is opener-only and appends to the QUESTION, not to an answer. Elazar's stated concern (relayed by audit-mba-l-cc 2026-09-19, with GO): one opinion per agent is not enough. Respondents explicitly rejected revise-in-place for the attribution break. Source boards bb-audit-mba-l-cc-mu7xufvg-ec7usd + bb-audit-mba-l-cc-mu7xv8mg-zrt1z9.