feat(short): la paleta de audio viva — GET /audio en el contrato, el prompt y el bucle de edición

shortsmith ya compone la banda sonora (sonar); ahora publica una paleta
(pulse, static) y este repo la consume en vivo: el prompt la ofrece con sus
notas de mood, validate_spec la usa como fuente de verdad para audio.preset,
y editar el preset en /short_spec es la manera gratis de escucharlas. Sin
/audio (404 o caída) todo cae a la paleta base y nada deja de renderizar.

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

README.md

@@ -92,6 +92,13 @@ saves the edited spec as a new output, and renders. No LLM in that path: it is
 free. The session comes from the filename, so it works even if the chat has
 researched something else since.
 
+**Soundtrack:** every Short carries a synthesized score — shortsmith composes
+it deterministically, no samples, no licensing. The palette comes live from
+`GET /audio` (the audio half of what `GET /templates` does for shots): `sonar`
+for case files, `pulse` for debunks, `static` for document drops. The model
+picks one to match the narrative shape, and the cheapest way to audition them
+is the edit loop — change `audio.preset` in the spec file and re-send it.
+
 Full spec of the phase: `docs/shortsmith-phase2-spec.md`.
 
 ## YouTube (`/upload_short`)