Strip comments and header metadata on ODT export

Each file is now cleaned before conversion: HTML comments are removed
and everything above the configurable Draft marker (default
"### Rough Draft:") is dropped. A "# Title:" line becomes the chapter
heading and a "# Slug:" line becomes an italic caption beneath it.

- new preprocess module (strip_comments + header parse) with tests
- Chapter gains a slug rendered via a Chapter_20_Caption style, italic
  at both paragraph-style and character-run level
- draft_marker config setting + top-bar field
- export title priority: override > # Title: > first # heading > filename

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/odt.rs

@@ -19,6 +19,8 @@ use zip::write::SimpleFileOptions;
 /// One chapter to export.
 pub struct Chapter {
     pub title: String,
+    /// Optional caption shown beneath the chapter title (from the `# Slug:` line).
+    pub slug: Option<String>,
     pub markdown: String,
 }
 
@@ -387,6 +389,10 @@ fn styles_xml() -> String {
    <style:text-properties fo:font-size="14pt" fo:font-weight="bold"/>
   </style:style>
   {headings}
+  <style:style style:name="Chapter_20_Caption" style:display-name="Chapter Caption" style:family="paragraph" style:parent-style-name="Standard" style:next-style-name="Text_20_body" style:class="text">
+   <style:paragraph-properties fo:margin-top="0in" fo:margin-bottom="0.2in"/>
+   <style:text-properties fo:font-style="italic" fo:font-size="12pt" fo:color="#666666"/>
+  </style:style>
   <style:style style:name="Quotations" style:family="paragraph" style:parent-style-name="Standard" style:class="html">
    <style:paragraph-properties fo:margin-left="0.4in" fo:margin-right="0.4in" fo:margin-top="0.05in" fo:margin-bottom="0.05in"/>
    <style:text-properties fo:font-style="italic"/>
@@ -448,6 +454,15 @@ fn content_xml(chapters: &[Chapter]) -> String {
             "<text:h text:style-name=\"Heading_20_1\" text:outline-level=\"1\">{}</text:h>",
             esc(&chapter.title)
         ));
+        if let Some(slug) = chapter.slug.as_deref().map(str::trim).filter(|s| !s.is_empty()) {
+            // Italic at both the paragraph-style and character-run level so the
+            // caption stays italic regardless of how a reader applies styles.
+            body.push_str(&format!(
+                "<text:p text:style-name=\"Chapter_20_Caption\">\
+                 <text:span text:style-name=\"T_italic\">{}</text:span></text:p>",
+                esc(slug)
+            ));
+        }
         body.push_str(&markdown_to_body(&chapter.markdown, 1));
     }
 
@@ -549,10 +564,12 @@ Done.";
         let chapters = vec![
             Chapter {
                 title: "Chapter One".to_string(),
+                slug: Some("the opening".to_string()),
                 markdown: "Hello **world**.\n\n## Scene\n\nText.".to_string(),
             },
             Chapter {
                 title: "Chapter Two".to_string(),
+                slug: None,
                 markdown: "- a\n- b\n".to_string(),
             },
         ];
@@ -576,5 +593,10 @@ Done.";
             .unwrap();
         assert!(content.contains(">Chapter One</text:h>"));
         assert!(content.contains(">Chapter Two</text:h>"));
+        // The slug renders as an italic caption paragraph under the first chapter.
+        assert!(content.contains(
+            "<text:p text:style-name=\"Chapter_20_Caption\">\
+             <text:span text:style-name=\"T_italic\">the opening</text:span></text:p>"
+        ));
     }
 }