fix(detect): Filter map hatching and Word internal bookmarks

[mlissner]

Summary

Fixes cross-hatch and TOC bookmark false positives from this 178-page filing (22 → 0 false detections) and this 555-page filing (1 → 0):

1. Cross-hatch color check — only dark-colored line patterns qualify as redaction cross-hatching. Map/chart hatching uses colored lines (salmon, blue, gray).

2. Cross-hatch line density cap — drawings with >100 lines are dense fill patterns (maps pack 776 lines into a 34x13pt area), not redaction X-hatching.

3. Cross-hatch minimum size — raised from 4pt to 6pt. Character-sized decorative patterns (5x7pt) were passing; real cross-hatching is 17pt+ tall.

4. Underscore bookmark filter — TOC entries starting with _ are Word internal bookmarks (_Hlk110177622, _References, _GoBack, etc.), not real headings.

Test plan

• test_map_hatching_not_cross_hatches with real-world map page
• test_small_crosshatch_decorations_no_results with real-world decorative patterns
• test_word_internal_bookmarks_no_results with synthetic PDF
• Real cross-hatch test still detects 16 redactions
• All 46 tests pass
• mypy and pre-commit pass
• Merge after fix(detect): Lower occlusion threshold from 80% to 75% #219 lands

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