- Add spine--format-defs defconst shared between index and book models
- Add spine-book-model with status extraction, format selector, notes, recommendation
- Add templates/book.mustache (Pico CSS, journal-style layout)
- Add httpd/book handler serving the detail page
- Link book titles in index view to /book?id=X
- Add spine-book-model-test.el with 13 tests
