docs(pdftract-14w0w): verify gap detection implementation complete

The detect_column_gaps function was already implemented in columns.rs with full test coverage. All acceptance criteria verified:
- 8 zeros < threshold: no gap
- 20 zeros middle: 1 gap detected
- Leading zeros >= threshold: gap emitted
- All-zero histogram: 0 gaps

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

notes/pdftract-14w0w.md

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+# pdftract-14w0w: Gap detection verification
+
+## Summary
+
+The `detect_column_gaps` function was already implemented in `crates/pdftract-core/src/layout/columns.rs` (lines 156-201). All acceptance criteria tests pass.
+
+## Implementation details
+
+The function:
+- Takes histogram slice and page_width
+- Calculates threshold: `(page_width * 0.03).ceil() as usize`
+- Returns `Vec<ColumnGap { lo, hi }>`
+
+Key behaviors:
+- Handles leading zeros (left margin) - emits gap if >= threshold
+- Handles trailing zeros (right margin) - emits gap if >= threshold  
+- Handles all-zero histogram (empty page) - returns no gaps
+- Handles empty histogram - returns no gaps
+
+## Acceptance criteria verification
+
+| Criterion | Test | Status |
+|-----------|------|--------|
+| 8 zeros, page_width=600: NO gap | `test_detect_column_gaps_short_zeros_no_gap` | PASS |
+| 20 zeros middle, page_width=600: 1 gap | `test_detect_column_gaps_middle_gap` | PASS |
+| Leading zeros >= threshold: 1 gap | `test_detect_column_gaps_leading_gap` | PASS |
+| All-zero histogram: 0 gaps | `test_detect_column_gaps_all_zeros_no_gaps` | PASS |
+
+Additional tests:
+- `test_detect_column_gaps_trailing_gap` - trailing margin gap
+- `test_detect_column_gaps_multiple_gaps` - multiple separated gaps
+- `test_detect_column_gaps_threshold_exact` - gap at exact threshold
+- `test_detect_column_gaps_threshold_minus_one` - gap just below threshold
+- `test_detect_column_gaps_empty_histogram` - empty input
+- `test_detect_column_gaps_no_zeros` - no gaps in histogram
+- `test_detect_column_gaps_small_page` - small page width
+- `test_detect_column_gaps_leading_and_trailing` - both margins
+
+All 36 column tests PASS (including 13 detect_column_gaps tests).
+
+## Files verified
+
+- `crates/pdftract-core/src/layout/columns.rs` - implementation (lines 89-201)
+- `crates/pdftract-core/src/layout/mod.rs` - exports ColumnGap, detect_column_gaps