feat(tools): add incremental org-mode HTML exporter

feat(tools): add incremental org-mode HTML exporter

Shared tool for the sibling hobby projects (qbasicapps, crtbasic,
physical, alyverkko-cli), whose "Update web site" scripts used to
re-export every page in one Emacs batch process per file.

- Re-exports only changed pages: per-page sha256 fingerprint covers
  the .org file plus its first-level #+INCLUDE dependencies (so
  editing an included .bas/SVG re-exports the including page)
- Runs all dirty exports in a single Emacs batch process with
  per-file error isolation (condition-case); --sequential for
  debugging, --force to rebuild everything
- Sentinel files (--sentinel, e.g. style.css) invalidate all pages
- State kept in a per-repo .org-export-state cache (gitignored)

Also correct the documented dependency version to 2.0.0.