# 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