feat: remove colon exclusion from bold heading detection

The colon exclusion was preventing legitimate headings like "Steps for
Using the Microscope:" and "Changing objectives:" from being detected.
The single edge case it was protecting (chart sub-headers) is less
impactful than the many headings it was blocking.

Benchmark improvement: MHS 0.51→0.52, MHS-S 0.61→0.62.

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

tests/snapshots/p1244-1996.md

@@ -8,7 +8,11 @@ Department of the Treasury **Internal Revenue Service**
 
 # and Report to Employer
 
-**This publication contains:** **Form 4070A, Employee’s Daily Record of** Tips **Form 4070, Employee’s Report of Tips to** Employer
+### This publication contains:
+
+### Form 4070A, Employee’s Daily Record of
+
+Tips **Form 4070, Employee’s Report of Tips to** Employer
 
 For the period
 

tests/snapshots/real-estate-pricing.md

@@ -8,7 +8,9 @@
 
 **Table I: Cap rate correlations**
 
-**Cap Rate Correlation With:*** **BBB Corp** **10-Year Bond Yield S&P Dividend** **Treasury (10-15 yr) Yield** Multifamily 0.187 0.771 0.068 Industrial-0.221 0.748-0.307 CBD Office-0.449 0.694-0.458 Retail-0.181 0.649-02.58
+## Cap Rate Correlation With:*
+
+**BBB Corp** **10-Year Bond Yield S&P Dividend** **Treasury (10-15 yr) Yield** Multifamily 0.187 0.771 0.068 Industrial-0.221 0.748-0.307 CBD Office-0.449 0.694-0.458 Retail-0.181 0.649-02.58
 
 * Based on 25 years of data for the 10-yrT & S&P DivYld; and 14 years for BBB.
 **Figure 1:** NCREIF cap rates vs. 10-yearTreasury