feat(short): narración — el grounding la cubre, el contrato la admite y el prompt la guía

El comprobador va primero, antes que el campo (fase 2 §12): la narración es
prosa que el modelo redacta, no una etiqueta que copia, y es donde se cuela
una cifra sin fuente. De paso, la huella de una cifra pasa a ser número +
unidad canónica: con la voz repitiendo la pantalla, '35,000 FT' y '35,000
feet' son el mismo dato y contarlos dos veces inflaría el informe del que
depende la revisión humana.

editorial_notes estima la duración CON la voz: la declarada es un suelo y sin
esto el modelo escribiría 40 s de shots, les colgaría narración y se enteraría
del Short de 65 s cuando ya está pagado.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/generator/shortspec.py

@@ -101,6 +101,31 @@ title a human reads, not a filename.
 - version is 1. Keep meta at 1080x1920.
 - audio.preset — pick the one whose mood fits the shape you chose:
 {presets}
+
+# 3b. Narration — the voice-over
+
+Every shot takes an optional `narration`: one or two spoken sentences, read \
+aloud by the renderer and burned in as captions. Write it for the ear.
+
+- **The first line is the whole hook.** Two seconds decide whether anyone \
+watches the rest, and the opening shot's narration is those two seconds. Lead \
+with the strangest true thing you have, not with a preamble.
+- Keep a line under 25 words. Long sentences lose the listener and stretch the \
+shot; the renderer will not cut your voice off, it will make the shot longer \
+instead, and a Short that drifts past 45 seconds is a Short people leave.
+- **Do not read the screen aloud.** The captions already show your words and \
+the template already shows its own. If the shot draws "35,000 FT", the voice \
+says what that altitude meant, not the number again.
+- Spoken register, not caption register: normal sentence case, ordinary \
+punctuation, whole words. The on-screen props are terse and uppercase; the \
+narration is a person talking. Write "seventeenth of November" rather than \
+"17 NOV" — the voice reads exactly what you type, and it will say "one seven \
+N-O-V" if you make it.
+- Not every shot needs one. Silence over a single strong image is a choice, \
+and a wall of continuous talking is not.
+- Everything in section 4 applies to narration word for word. It is prose you \
+compose rather than a label you copy, which makes it the easiest place to \
+slip in a figure no source gave you — and it is checked exactly like the rest.
 - The closing shot carries the domain, uppercase, no protocol: {domain}
 
 # 4. Grounding — this is the part that matters