feat: strip repeated headers/footers from markdown output

Add strip_repeated_lines() preprocessing step that removes running
headers and footers before markdown conversion. Uses multiple guard
rails to avoid false positives:

- Edge-line detection: only considers lines among the first/last 3
  distinct Y positions on each page (not percentage-based margins)
- Y-position consistency: requires low variance across pages (stddev
  < 5% of page span) to distinguish headers from scattered table content
- Position-aware removal: only strips instances at page edges, preserving
  body content that happens to match a header/footer text
- Skips structural lines (headings, list items), short lines (<10 chars),
  and decorative separators (repeated single characters)
- Frequency threshold: >= max(3, page_count * 30%) distinct pages

New strip_headers_footers option on MarkdownOptions (default: true).

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

tests/snapshots/p1244-1996.md

@@ -33,28 +33,19 @@ Date Date **a. Tips received**
 **b. Credit card tips c. Tips paid out to d. Names of employees to whom you**
 tips of directly from customers received other employees paid tips rec’d. entry and other employees 1 2 3 4 5 **Subtotals** **For Paperwork Reduction Act Notice, see Instructions on the back of Form 4070. Page 1**
 
-Date Date **a. Tips received**
-
-**b. Credit card tips c. Tips paid out to d. Names of employees to whom you**
-tips of directly from customers received other employees paid tips rec’d. entry and other employees
+received other employees paid tips rec’d. entry and other employees
 
 7 8 9 10 11 12 13 14 15 **Subtotals**
 
 **Page 2**
 
-Date Date **a. Tips received**
-
-**b. Credit card tips c. Tips paid out to d. Names of employees to whom you**
-tips of directly from customers received other employees paid tips rec’d. entry and other employees
+received other employees paid tips rec’d. entry and other employees
 
 17 18 19 20 21 22 23 24 25 **Subtotals**
 
 **Page 3**
 
-Date Date **a. Tips received**
-
-**b. Credit card tips c. Tips paid out to d. Names of employees to whom you**
-tips of directly from customers received other employees paid tips rec’d. entry and other employees
+received other employees paid tips rec’d. entry and other employees
 
 27 28 29 30 31 **Subtotals** **from pages** **1, 2, and 3** **Totals**