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>

.gitignore

@@ -1,5 +1,10 @@
 .claude/
 node_modules/
 output/book.html
-# Keep the PDF in git if you want to track versions, otherwise add:
+output/cover.html
+output/book-meta.json
+# Interior PDF is large (~100 MB with placeholder pages) — excluded from git.
+# Re-add images and re-run `pnpm all` to regenerate.
 output/kaleidoskop.pdf
+# Cover PDF is small (~140 KB) — tracked in git.
+# output/cover.pdf