Review form: preventive cue under Calificación when Rectificar is selected and the grade is empty

Measured 2026-08-15..22 from appEvents category=toast: 13 'Ingresá una calificación entre 0 y 10.' error toasts from 6 distinct teachers on /practicas/[id], desktop Chrome and Android; each cluster lands seconds before a RECTIFICAR practicaReviews row (e.g. 8ecaf163 toasts 21:27:48/21:28:31 -> rectificar 21:28:47; b6cbdb54 22:29:38/42 -> rectificar 22:29:56; 58546e24 13:15:39 -> rectificar 13:17:03). Teachers pick Rectificar, submit without a grade (pre-2026-07-17 behaviour), get blocked, then enter one. Remedy per evolutiva-coder-commons §No Silent Failures: a PREVENTIVE VISUAL CUE, never a relaxed rule — inline hint under the grade field, rendered only when isRectificar && grade empty. The 07-17 rule (grade required on every verdict incl. rectificar) is unchanged. Separate product question for Elazar, NOT decided here: whether a rectificar should carry a 0-10 grade at all — the stored values (8, 7, 5 on send-backs) are pedagogically odd.