onlyworlds 0.6.0

• Export a world folder to Open Knowledge Format (OKF) v0.2 (onlyworlds.okf, Keel #191): export_okf(folder, out_dir) or python -m onlyworlds.okf FOLDER OUT-DIR writes one markdown concept per element (fields in the frontmatter, text under headings, links as bundle paths and as markdown links), the World as world.md, and an index per type. The same folder gives the same bytes. Nothing is left out in silence: a file the reader skipped, a key that is not a field of its type, or a link that is not an id or points at the wrong type is a note in the returned OkfReport (stderr from the command line). Ported from schema-dist's walk/okf_export.py, it reads through read_folder and the generated schema tables. It differs from the branch on purpose: no generated actor (OKF §7 asks for a human: actor on hand-written words, and a folder does not say who wrote them), in-prose ow:// references become links into the bundle, and a text line that already begins with a backslash gets one more, so every escaped line reads back.
• PyYAML only for OKF: the export needs the new okf extra (pip install "onlyworlds[okf]"). import onlyworlds still imports nothing outside the standard library.
• The schema module grows (same pin, v0.32.0-dist.23): BASE_FIELDS, WORLD_FIELDS, SECTIONS (each type's fields under their schema groups, in order), LINK_TARGETS, GENERIC_LINKS and EXTENSION_PREFIXES, generated from the vendored walk like the rest and checked against FIELD_KINDS and GENERIC_PAIRS at generation.