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>

src/generator/shortspec.py

@@ -98,7 +98,9 @@ and your whole answer becomes unparseable. This is the single most common way \
 this task fails.
 - meta.id is a lowercase slug (letters, digits, - and _). meta.title is the \
 title a human reads, not a filename.
-- version is 1. Keep meta at 1080x1920 and audio preset "sonar".
+- version is 1. Keep meta at 1080x1920.
+- audio.preset — pick the one whose mood fits the shape you chose:
+{presets}
 - The closing shot carries the domain, uppercase, no protocol: {domain}
 
 # 4. Grounding — this is the part that matters
@@ -144,6 +146,14 @@ Return the JSON object now."""
 
 PALETTE_FALLBACK = "ink, amber, amber_dark, muted, dim, red"
 
+#: Si el caller no trae la paleta de audio, el prompt solo ofrece lo que
+#: cualquier shortsmith renderiza.
+PRESETS_FALLBACK = {"sonar": "low drone and sonar pings", "none": "digital silence"}
+
+
+def _describe_presets(presets: dict[str, str]) -> str:
+    return "\n".join(f'  "{name}" — {note}' for name, note in sorted(presets.items()))
+
 
 @dataclass
 class SpecResult:
@@ -305,12 +315,14 @@ LLMCall = Callable[[str, str], Awaitable[str]]
 
 class ShortSpecWriter:
     def __init__(self, llm_call: LLMCall, templates: dict[str, dict],
-                 refresh_templates: Optional[Callable[[], Awaitable[dict]]] = None):
+                 refresh_templates: Optional[Callable[[], Awaitable[dict]]] = None,
+                 presets: Optional[dict[str, str]] = None):
         self.llm_call = llm_call
         self.templates = templates
         #: Se vuelve a pedir el contrato si una validación falla: el
         #: renderizador puede haberse actualizado a mitad de la run.
         self.refresh_templates = refresh_templates
+        self.presets = presets or dict(PRESETS_FALLBACK)
 
     def build_prompt(self, topic: str, context: str, article_url: Optional[str],
                      domain: str) -> str:
@@ -322,6 +334,7 @@ class ShortSpecWriter:
             shapes=NARRATIVE_SHAPES,
             templates=describe_templates(self.templates),
             colors=_palette(self.templates),
+            presets=_describe_presets(self.presets),
             domain=domain,
             target_min=TARGET_MIN_DURATION,
             target_max=TARGET_MAX_DURATION,
@@ -359,7 +372,7 @@ class ShortSpecWriter:
 
             last_spec = spec
             try:
-                validate_spec(spec, self.templates)
+                validate_spec(spec, self.templates, presets=self.presets)
             except SpecInvalid as e:
                 history.append(e.errors)
                 feedback = _format_errors(e.errors)