/api/slide-decompose — non-destructive stub generator for the Phase 1 → Phase 2 transition

Dev-only API route that writes an empty per-slide stub file at `{slidesComponentsRoot}/{variant}/{slot}-{slug}.astro` in the consumer's working tree. Non-destructive: refuses to overwrite, returns 409 if the file already exists. The stub is a minimal `<section data-slot data-variant></section>` with a generated-by header comment instructing the recreate-do-not-extract discipline (Phase 2 of the deck-iteration-workflow). Static Vercel builds never carry this route; file writes only happen against a local working tree.