feat(extractor): geometric underline detection on TextItem (#116)

* feat(extractor): geometric underline detection on TextItem (ENG-5015)

PDFs carry no underline font flag — underlines are stroked horizontal
lines or thin filled rects drawn under the baseline. Correlate those
graphics (already parsed from the content stream) with text items in a
post-pass: a rule within ~0.35em below the baseline covering >=60% of
an item's width marks is_underline.

Exposed through the napi and python bindings. Verified on real docs:
4/4 underlined sentences flagged on a Japanese report, links/headings
flagged on 8 of 10 underline-bearing eval docs, zero flags on docs
without underlines. Known FP source (table cell borders) documented —
downstream applies inline styling only to plain-text regions.

napi 1.9.8 -> 1.9.9.

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix(extractor): underline rules only from painted rects, normalized extents (review)

Two review fixes: (1) normalize rect extents before the thickness/width
checks — `re` operands pass through the CTM so width/height can be
negative, which missed negative-width rules and let negative-height
bands pass as thin; (2) only feed painted rects to underline detection —
`re` rects now wait in a pending list until a paint operator (S/s, f/F/
f*, B/B*/b/b*) confirms them, and `re W n` clip-only paths are discarded
at `n`, so invisible clip boundaries no longer underline nearby text.
Marking moved into content_stream where paint state lives (pre-rotation,
consistent device space).

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix(extractor): harden underline detection

* feat(cli): export positioned text item json

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

@@ -111,6 +111,9 @@ pdf2md document.pdf
 # JSON output (for piping)
 pdf2md document.pdf --json
 
+# Positioned TextItem JSON, including is_underline metadata
+pdf2md document.pdf --items-json
+
 # Raw markdown only (no headers)
 pdf2md document.pdf --raw