fix(headers): Reduce false heading detection and merge split headings

Three changes to improve heading quality:

1. Raise heading threshold from 1.1x to 1.2x base font size, reducing
   false positives where slightly larger body text was promoted to headers.

2. Add word count guard (max 15 words) to skip heading detection for
   long body paragraphs that happen to use a larger font.

3. Add merge_heading_lines() preprocessing that joins consecutive lines
   at the same heading level on the same page (e.g., "About Glenair,
   the Mission-Critical" + "Interconnect Company" → single heading).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/integration_tests.rs

@@ -695,12 +695,12 @@ fn test_header_level_h3() {
 #[test]
 fn test_header_level_h4() {
     use pdf_inspector::markdown::to_markdown_from_items;
-    // Four heading tiers: 24.0 → H1, 18.0 → H2, 15.0 → H3, 13.5 → H4
+    // Four heading tiers: 24.0 → H1, 18.0 → H2, 15.0 → H3, 14.5 → H4
     let items = vec![
         make_text_item("H1 Title", 100.0, 850.0, 24.0, 1),
         make_text_item("H2 Title", 100.0, 800.0, 18.0, 1),
         make_text_item("H3 Title", 100.0, 750.0, 15.0, 1),
-        make_text_item("H4 Title", 100.0, 700.0, 13.5, 1),
+        make_text_item("H4 Title", 100.0, 700.0, 14.5, 1),
         make_text_item("body text one", 100.0, 650.0, 12.0, 1),
         make_text_item("body text two", 100.0, 630.0, 12.0, 1),
         make_text_item("body text three", 100.0, 610.0, 12.0, 1),