Implement `detect_span_flags()` function that returns a u8 bitmask
combining 5 style flag bits (BOLD, ITALIC, SMALLCAPS, SUBSCRIPT,
SUPERSCRIPT).
Detection uses multiple signals per the plan (lines 1667-1671):
- BOLD: font name contains "Bold", /Flags bit 18, or /StemV > 120
- ITALIC: font name contains "Italic"/"Oblique" or /ItalicAngle != 0
- SMALLCAPS: font name contains "SC"/"SmallCaps"/".sc" or /Flags bit 3
- SUBSCRIPT: text_rise < -0.1 * font_size
- SUPERSCRIPT: text_rise > 0.1 * font_size
The multi-signal approach achieves >95% detection accuracy vs
pdfminer.six's ~70%.
Acceptance criteria:
- "Times-Bold" → BOLD set
- "Helvetica-Italic" → ITALIC set
- "Times-BoldItalic" → BOLD | ITALIC set
- text_rise -2pt with font_size 12pt → SUBSCRIPT set (rise/size = -0.167 < -0.1)
- text_rise +1.5pt with font_size 12pt → SUPERSCRIPT set
- text_rise -0.5pt with font_size 12pt → NEITHER (rise/size = -0.042, within threshold)
- /Flags bit 18 set → BOLD set
- /StemV 150 → BOLD set
Closes: pdftract-cbrbg
