feat(web-video-presentation): optional slides.ts + slide-data.json export

[dy2651]

Summary

Add an optional slides.ts data layer to the web-video-presentation skill and a slide-data.json export, so presentations can be consumed/edited by a generic web viewer/editor (not just the React recording project).

What changed

• templates/src/registry/types.ts: new SlideStepType (14 named types + custom), SlideStepMeta (loose string | string[] field bag), ChapterSlidesSegment, SlideDataExport; ChapterDef.slides? optional field with length-lock docs.
• templates/src/chapters/01-example/slides.ts (new): example slides declaration (hero/fact/highlight) matching the existing example narrations.
• templates/scripts/export-slide-data.ts (new): reads each chapter's narrations.ts (required) + optional slides.ts, merges them into a single slide-data.json at project root. Drift guard: if slides.length !== narrations.length it throws.
• templates/src/registry/slide-config.ts (new): top-level { title, subtitle } source.
• templates/src/registry/chapters.ts: wire example chapter's slides + doc comments.
• scripts/scaffold.sh: copy the new templates, add export-slide-data npm script, mention in completion notes.
• SKILL.md / references/CHAPTER-CRAFT.md: document the slides.ts convention (length-lock, supported types, optional fallback).

Why optional / backward compatible

Chapters without slides.ts still export fine — the script auto-infers a coarse step type per narration (empty→transition, index 0→hero, numbers→study, question marks→question, short→highlight, else fact). So existing projects don't break; the real visual structure is only editable in a web editor when slides.ts is present.

Usage

cd presentation
npm run export-slide-data            # merge chapters + slides + narrations → slide-data.json
npm run export-slide-data -- --print # also print to stdout

Verified end-to-end: scaffold → add slides.ts → export → 3-step demo produces valid slide-data.json; missing-slides chapter auto-infers; length mismatch (4 vs 3) throws the drift-guard error.