DecomposeFirstPlaceholder — empty-slot fallback rendered when a per-slide file doesn't exist yet

Rendered by `/play/[deck]/[variant]/[slot]` when the resolved per-slide file at `{slidesComponentsRoot}/{variant}/{slot}-{slug}.astro` does not exist. Communicates the next action — open the TOC at `?focus={slot}`, rank as redo-worthy, click scaffold to create the stub — without sounding like an error state. The instructive-helpful tone is deliberate: this is the framework's invitation to begin the rank → decompose → recreate loop, not an Astro 404.