Sync opam package metadata including x-maintenance-intent
and external dependency specifications.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

- Normalize slugs to match Jekyll behavior (dots → dashes)
so geotessera-python-0.7 becomes geotessera-python-0-7

- Add filesystem PDF check to JSON feed attachments
matching arod_papers.ml behavior for static/papers/*.pdf

- Add DOI cache infrastructure for external references:
- New bushel_doi_entry.ml module for parsing data/doi.yml
- Load DOI entries in bushel_loader.ml
- Complete note_references with external DOI URL scanning
- Scan for doi.org URLs and publisher URLs (Elsevier, IEEE, etc.)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

- atp: add re dependency
- frontmatter: add yamlt dependency
- init: convert bytesrw, eio, bytesrw-eio from depopts to real deps
- mdns: add cstruct, fmt, cmdliner, eio_main dependencies
- owntracks: add geojson, bytesrw dependencies
- tomlt: convert eio, jsont from depopts to real deps, remove (optional) from sublibraries

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>