Advance a single confirmed row to the next workflow stage.
Preconditions:
Caller must be able to edit this row at its current stage
Row must be confirmed (or locked) — same gate as dashboard’s bulk
advance, prevents pushing unreviewed work forward
On success:
row.stage_index += 1 (capped at total_stages — postreview)
is_confirmed cleared so the next stage’s reviewer starts fresh
emits ROW_STAGE_ADVANCED so other browsers / dashboard refresh
Per-file workflow note: this endpoint refuses per_file projects because
in that mode rows advance together as a target, not individually. The
sidebar should hide the Advance button when workflow_mode == ‘per_file’.