Add KDP cover PDF generator

scripts/cover.js computes the exact cover canvas dimensions from the
page count in book-meta.json (written by build.js), using the KDP
Premium Color spine formula (0.002347 in/page), and renders a
Nunjucks template to a single PDF containing back cover, spine, and
front cover with bleed (0.125 in) and safe-zone overlay guides.

- `pnpm cover`  — generate output/cover.pdf
- `pnpm all`    — build interior + both PDFs in one command
- Cover artwork slots: images/cover/front.png, images/cover/back.png

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -6,7 +6,9 @@
   "scripts": {
     "build": "node scripts/build.js",
     "pdf": "node scripts/pdf.js",
+    "cover": "node scripts/cover.js",
     "book": "pnpm build && pnpm pdf",
+    "all": "pnpm build && pnpm pdf && pnpm cover",
     "watch": "node --watch scripts/build.js"
   },
   "pnpm": {