fix(headings): digit-only lines do not define heading tiers (#166)

* fix(headings): digit-only lines do not define heading tiers

A large bold page number (14pt folio over 11pt body) claimed tier 0:
every real heading demoted one level document-wide, and the bold-size
fallback (which requires an empty tier list) was blocked for documents
whose headings match body size.

Bench-neutral (MHS scores relative hierarchy); pdf-evals: 18 docs get
their heading levels back (#### -> ###), semantic composite +0.0006,
no percentile down.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(headings): exclude digit-only lines from the bold fallback tier pass too

The exclusion in the main pass wasn't enough: with the page-number
tier gone, the bold fallback re-collected the same bold folio. Also
regenerates the thermo-freon12 snapshot (cosmetic churn on the
scrambled legend fixture).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/markdown/analysis.rs

@@ -374,6 +374,15 @@ pub(crate) fn compute_heading_tiers(lines: &[TextLine], base_size: f32) -> Vec<f
     for line in lines {
         if let Some(first) = line.items.first() {
             if first.font_size / base_size >= 1.2 {
+                // Digit-only lines (page numbers, issue numbers) must not
+                // define heading tiers: a large bold folio claims tier 0 and
+                // blocks the bold-size fallback for the document's real
+                // same-size headings.
+                let text = line.text();
+                let t = text.trim();
+                if !t.is_empty() && t.chars().all(|c| !c.is_alphabetic()) {
+                    continue;
+                }
                 heading_sizes.push(first.font_size);
             }
         }
@@ -398,6 +407,11 @@ pub(crate) fn compute_heading_tiers(lines: &[TextLine], base_size: f32) -> Vec<f
     if tiers.is_empty() {
         let mut bold_sizes: Vec<f32> = lines
             .iter()
+            .filter(|line| {
+                let text = line.text();
+                let t = text.trim();
+                !t.is_empty() && t.chars().any(|c| c.is_alphabetic())
+            })
             .filter_map(|line| line.items.first())
             .filter(|it| it.is_bold && it.font_size / base_size >= 1.05)
             .map(|it| it.font_size)
@@ -508,6 +522,19 @@ mod tests {
         }
     }
 
+    #[test]
+    fn digit_only_lines_do_not_define_tiers() {
+        // A 14pt bold page number must not claim tier 0 — that both demotes
+        // every real heading a level and blocks the bold-size fallback.
+        let lines = vec![
+            line_of("76", 14.0, true, 760.0),
+            line_of("Replace", 11.0, true, 700.0),
+            line_of("body text at eleven points", 11.0, false, 680.0),
+        ];
+        let tiers = compute_heading_tiers(&lines, 11.0);
+        assert!(tiers.is_empty(), "page number claimed a tier: {tiers:?}");
+    }
+
     #[test]
     fn bold_fallback_tiers_when_nothing_clears_ratio_gate() {
         // 10pt body, 11pt bold section headings (book-style): no size clears

tests/snapshots/thermo-freon12.md

@@ -6,9 +6,7 @@
 
 #### Thermodynamic Properties
 
-**of**
+**of** ®
-
-®
 
 # Freon 12