chunkmd, mdscrub: the chapter a converted book is actually divided by

Two things between a converted EPUB and its chapter titles.

The heading level taken as the chapter map was the shallowest present. Pandoc
writes the book's title as the only `#` and every real chapter as a `##`, so
the shallowest level had one heading in it and the whole book came back as a
single chapter. The dividing level is now the shallowest one used more than
once — a level used once is a title.

And an empty bracketed span survived the heading cleaner, because the pattern
wanted at least one character between the brackets. Pandoc writes an anchor
as exactly that: `## []{#link2H_INTR} Introduction`.

Gibbon's first volume, converted from the Project Gutenberg EPUB: 2 distinct
chapter titles before, 61 after, and none of them carrying an element id.

examples/rust/Cargo.toml

@@ -0,0 +1,16 @@
+# Standalone so that `cargo run` here does not need the workspace above it.
+[workspace]
+
+[package]
+name = "catena-example"
+version = "0.0.0"
+edition = "2024"
+rust-version = "1.85"
+publish = false
+
+[dependencies]
+booksniff = { path = "../../crates/booksniff" }
+chunkmd = { path = "../../crates/chunkmd" }
+citefind = { path = "../../crates/citefind" }
+corpus-core = { path = "../../crates/corpus-core" }
+mdscrub = { path = "../../crates/mdscrub" }