fix(layout): unfuse independent column runs sharing a baseline (#163)

* fix(layout): unfuse independent column runs sharing a baseline

Two-column report pages with charts fused headings into the adjacent
column's body text: the columns' ~6pt gutter is below what histogram
valley detection can safely use, so the page grouped single-column and
same-baseline items from both columns joined into one line ('6.2.
Expectations for Re-Hiring Employees' + mid-sentence text, killing
MHS and NID on the whole survey-report doc family).

Three changes:
- Line grouping splits same-baseline runs separated by a wide void
  (>3x font size, >=30pt) when the incoming run starts lowercase
  (mid-sentence continuation from another column) and both sides are
  multi-word prose. TOC page numbers, dot leaders, and table cells
  (numbered/capitalized) stay joined.
- Column detection is blind to chart-region text (tight 2pt bounds —
  wider padding ate rows adjacent to charts), via a chart-aware line
  grouping variant wired from the markdown pipeline.
- validate_and_build_columns computes its vertical span from
  histogram-eligible items only, so full-width captions no longer sink
  the overlap ratio for partial-page column regions.

opendataloader-bench: overall 0.8532 -> 0.8554, MHS 0.761 -> 0.769;
doc 038 +0.434, no regressions. pdf-evals: 34 snapshots change,
semantic composite wash (0.5749 -> 0.5748), no per-doc mover >0.015.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(layout): review follow-ups — chart-aware band-split grouping, single chart scan per page

Band-split pages now route through the chart-aware grouping too, and
the band loop reuses the precomputed page_chart_map instead of
re-scanning the rect list per page.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/extractor/mod.rs

@@ -28,6 +28,7 @@ pub(crate) use fonts::FontStyleCache;
 pub(crate) use layout::detect_columns;
 pub use layout::group_into_lines;
 pub(crate) use layout::group_into_lines_with_thresholds;
+pub(crate) use layout::group_into_lines_with_thresholds_and_charts;
 pub(crate) use layout::is_newspaper_layout;
 pub(crate) use layout::ColumnRegion;