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/include/catena.h

@@ -98,6 +98,10 @@ char *catena_series_of(const char *title, const char *author);
  * one — an answer, not an error. */
 char *catena_parse_identifier(const char *text);
 
+/* Whether a chunk of body text carries footnote or endnote citations.
+ * Returns 1 for yes, 0 for no, -1 on error. */
+int catena_looks_like_footnote_citations(const char *text);
+
 /* Read the references out of a bibliography or a page of notes. Returns a
  * JSON array, one object per reference. */
 char *catena_find_citations(const char *text);