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.

crates/mdscrub/src/heading.rs

@@ -18,8 +18,10 @@ fn pattern(source: &str) -> Regex {
 /// `[text](url)` — a heading wrapped in a link to its own anchor.
 static LINK: LazyLock<Regex> = LazyLock::new(|| pattern(r"\[([^\]]*)\]\([^)]*\)"));
 
-/// `[text]{.cls}` — pandoc's bracketed inline span.
-static BRACKETED: LazyLock<Regex> = LazyLock::new(|| pattern(r"\[([^\]]+)\]\{[^}]*\}"));
+/// `[text]{.cls}` — pandoc's bracketed inline span. The text may be empty:
+/// `[]{#chapter-3}` is how an anchor with nothing in it arrives, and a
+/// heading that keeps one is a chapter title with an element id in it.
+static BRACKETED: LazyLock<Regex> = LazyLock::new(|| pattern(r"\[([^\]]*)\]\{[^}]*\}"));
 
 /// A trailing `{.attr}` block.
 static ATTRIBUTE_TAIL: LazyLock<Regex> =