feat: detect bold-only lines as section headings

Bold lines at body font size that are standalone (preceded by a paragraph
break) and have ≥3 words are promoted to headings. This catches the
common pattern in academic/technical PDFs where section headings use
bold text at the same size as body text.

Guards against false positives: minimum word count, colon-ending
exclusion (labels like "Table I:").

Benchmark improvement: MHS 0.37→0.50, overall 0.71→0.75.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/snapshots/real-estate-pricing.md

@@ -22,7 +22,9 @@ R E V I E W 8 5
 
 **Figure 2:** Capratespreadsover10-yearTreasury
 
-**Basis Points -200** -400
+## Basis Points -200
+
+-400
 
 -600
 
@@ -36,7 +38,9 @@ R E V I E W 8 5
 
 **Table II: Correlationsofspreadsbypropertytype**
 
-**Correlation of Cap Rate Spreads Over Treasury** **Multifamily Industrial CBD Office**
+## Correlation of Cap Rate Spreads Over Treasury
+
+## Multifamily Industrial CBD Office
 
 ||Multifamily|Industrial|CBD Office|
 |---|---|---|---|