A build_logs row is opened with finished_at NULL and closed only by the
process that runs the build. If hive-c0re stopped in between, the row stayed
open for good: the dashboard listed it with no status, its log stream never
sent `done`, and the retention sweep, which skips open rows, never deleted it.
Coordinator::open now closes every open row as `fail` and appends a line
saying hive-c0re stopped. It runs before this process starts any build, and
only there: BuildLogs::open itself leaves rows alone so a second opener can't
close another process's live rows.
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