bindings: what a closed subject scheme says about a heading

book-pipeline held its own copy of the scheme operations bibclean already
had. The scheme itself — which headings a collection uses, their aliases,
their parents — stays with the collection, because that is the most
collection-specific thing there is. What a scheme *means* is the same
question everywhere, and this answers it: `canonical_subject`,
`compose_subjects`, `needs_subject`.

Checked against the Python over the library's 228 distinct tags and all
5,385 books: identical on every one.

crates/bindings/python/catena/__init__.py

@@ -13,14 +13,17 @@ lines of JSON.
 from ._catena import (
     __version__,
     cue_windows,
+    canonical_subject,
     chunk_markdown,
     citation_prompt,
+    compose_subjects,
     count_tokens,
     find_citations,
     fuse_hits,
     identify,
     looks_like_footnote_citations,
     merge_citation_reply,
+    needs_subject,
     pack_cues,
     parse_identifier,
     parse_subtitles,
@@ -34,14 +37,17 @@ from ._catena import (
 __all__ = [
     "__version__",
     "cue_windows",
+    "canonical_subject",
     "chunk_markdown",
     "citation_prompt",
+    "compose_subjects",
     "count_tokens",
     "find_citations",
     "fuse_hits",
     "identify",
     "looks_like_footnote_citations",
     "merge_citation_reply",
+    "needs_subject",
     "pack_cues",
     "parse_identifier",
     "parse_subtitles",