feat: lookahead-based isolated line heading detection

Pre-scan lines to identify "isolated" ones — short lines (1-6 words)
with paragraph breaks both before AND after. These are heading
candidates even at body font size, common in academic papers
("Acknowledgements", "Limitations", "B.3 Prompt Engineering").

Inspired by opendataloader's HeadingProcessor which passes prevNode
and nextNode context to the heading probability scorer.

The isolated signal (+0.3) combines with rarity/bold/standalone
signals. A per-page density guard prevents false positives on
multi-column pages where many lines appear isolated. Continuation
word detection (ending in "the", "and", etc.) filters wrapped
paragraph lines.

MHS=0 docs: 18→13. MHS-S +0.004. No regressions.

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

tests/snapshots/p1244-1996.md

@@ -76,7 +76,7 @@ forms simpler, we would be happy to hear from you. You can write to the Tax Form
 
 **Unreported Tips.—If you received tips of $20 or** more for any month while working for one employer but did not report them to your employer, you must figure and pay social security and Medicare taxes on the unreported tips when you file your tax return. If you have unreported tips, you must use Form 1040 and Form 4137, Social Security and Medicare Tax on Unreported Tip Income, to report them. You may not use Form 1040A or 1040EZ. Employees subject to the Railroad Retirement Tax Act cannot use Form 4137 to pay railroad retirement tax on unreported tips. To get railroad retirement credit, you must report tips to your employer. If you do not report tips to your employer as required, you may be charged a penalty of 50% of the social security and Medicare taxes (or railroad retirement tax) due on the unreported tips unless there was reasonable cause for not reporting them. **Additional Information.—Get Pub. 531, Reporting** Tip Income, and Form 4137 for more information on tips. If you are an employee of certain large food or beverage establishments, see Pub. 531 for tip allocation rules. **Recordkeeping.—If you do not keep a daily** record of tips, you must keep other reliable proof of the tip income you received. This proof includes copies of restaurant bills and credit card charges that show amounts customers added as tips. Keep your tip income records for as long as the information on them may be needed in the administration of any Internal Revenue law.
 
-**Instructions (continued)**
+### Instructions (continued)
 
 Use this space to total your tips for the year
 

tests/snapshots/thermo-freon12.md

@@ -1,8 +1,8 @@
-**Technical Information**
+##### Technical Information
 
 ## l T-12 SI
 
-DuPont Fluorochemicals
+##### DuPont Fluorochemicals
 
 #### Thermodynamic Properties
 
@@ -20,11 +20,11 @@ Tables of the thermodynamic **Units** properties of R-12 have been developed and
 
 S.A., Lemmon, E.W., and Peskin, Vf = Fluid (liquid) specific volume
 A.P., NIST Standard Reference in cubic meters per kilogram Database 23, NIST thermodynamic and transport properties of Vg = Vapour (gas) specific volume refrigerants and refrigerant in cubic meters per kilogram mixtures – REFPROP version 6.01, Standard Reference Data Program, df and dg = Fluid and Vapour National Institute of Standards and (respectively) densities in Technology, 1998). kilograms per cubic meter
-H = Enthalpy (kJ/kg)
+##### H = Enthalpy (kJ/kg)
 
-S = Entropy (kJ/kg.K)
+##### S = Entropy (kJ/kg.K)
 
-**Physical Properties**
+##### Physical Properties
 
 |Chemical Formula|CCl2F2|
 |---|---|