feat(short): el prompt sabe cuánto texto cabe dibujado, y el informe grita si quedó ilegible

x-fits llega vivo del contrato y se pone delante del modelo: es el único
límite que nada rechaza, porque el renderizador encoge en vez de fallar. Sin
esto el modelo escribe una cita de 58 caracteres para un hueco de 16 y se
entera cuando ya está renderizada.

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

src/generator/shortspec.py

@@ -90,6 +90,12 @@ not a target. Aim for {target_min:.0f}-{target_max:.0f}.
 a card with four rows needs longer than a headline.
 - Every string is drawn as given. Write them the way they should appear: \
 SHORT, UPPERCASE, no trailing punctuation. A headline is 2-5 words.
+- **"CABE ~N caracteres dibujados" is a width, and it is the one limit nothing \
+will catch for you.** Nothing rejects a longer string: the renderer shrinks the \
+type until it fits, so a quote of 58 characters in a 16-character slot is drawn \
+at a quarter of its size and ends up the smallest text on a frame it was \
+supposed to dominate. Stay at or under N. On a quote that means picking a \
+shorter verbatim span, never squeezing the whole sentence in.
 - Respect every max length and list-length limit above. They are enforced.
 - Colours are palette names ({colors}) — never hex.
 - Quotes carry the typographic quote marks: “SPLIT RADAR IMAGE”, with U+201C \