bindings: the footnote signal, which book-pipeline still had its own copy of

`looks_like_footnote_citations` is the function chunkmd's `looks_like_citations`
was ported from, and book-pipeline was still running the Python original at
four call sites. Bound on all four surfaces so it can stop.

Checked against the Python over 34,458 chunks of the live corpus: no
disagreement.

crates/bindings/README.md

@@ -79,6 +79,7 @@ except ValueError as why:
 | `series_of` | [`seriesof`](../seriesof/README.md) |
 | `parse_identifier` | [`idnorm`](../idnorm/README.md) |
 | `find_citations` | [`citefind`](../citefind/README.md) |
+| `looks_like_footnote_citations` | whether a body chunk carries notes worth reading |
 | `citation_prompt` | the question `citefind --prompt` writes |
 | `merge_citation_reply` | what `citefind --merge` does with the answer |