Edit one table-read line.

Partial update. Only the named fields change; the line's current values are saved to its history first (versions); the rest are left alone. There is deliberately no speed and no acting-direction field — a re-voice of the line (revoice_line, POST /artifacts/{artifactId}/revoice-line) sets each, so a volume change keeps the line's existing speed and no edit touches its direction (and there is no emotion field: a line is performed from its acting direction); actingDirection here is refused with 400 validation_failed, naming that re-voice. An unknown field, a wrong-typed value or an index past the end is refused with 400 validation_failed — the same validation the Writers Lab's line editor, MCP edit_script_line and sleeperhit script line set go through. Answers the line as it now stands. Free and naturally idempotent, so no Idempotency-Key: it renders nothing, and the next recording of that line uses it.