fix(markdown): Render image-only pages in order and collapse dot leaders

Image-only pages (e.g. scanned cover pages) were appended at the end of
output instead of appearing in page order. Now flushes tables/images for
intermediate pages with no text lines during page transitions.

Also collapses dot-leader runs (4+ dots) into " ... " for cleaner TOC
rendering, and adds debug_pages binary for inspecting per-page items.

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

Cargo.toml

@@ -45,3 +45,7 @@ path = "src/bin/detect_pdf.rs"
 [[bin]]
 name = "debug_ygaps"
 path = "src/bin/debug_ygaps.rs"
+
+[[bin]]
+name = "debug_pages"
+path = "src/bin/debug_pages.rs"