Implement cluster_spans_into_lines for Phase 4.2 line formation.
Groups spans into lines by baseline proximity using threshold 0.5 * median_font_size.
- Add HasFontSize trait for types with font_size
- Implement cluster_spans_into_lines function
- Compute baseline for each span
- Sort by baseline ASC
- Sweep and cluster within threshold
- Emit Line per cluster
- Sort spans by x0 within each line
- Add finalize_line_cluster helper
- Export new items from layout module
Tests: All 11 acceptance criteria tests pass
- Spans baselines 100, 100.5, 105 with median 12: one line
- Spans baselines 100, 110 with median 12: two lines
- Superscript stays on same line as base text
- Empty input produces empty output
- Threshold is 0.5 * median_font_size (not hardcoded)
Closes: pdftract-6bwq4
