Adopt accessibility capture from pi-annotate patterns #23

pi-annotate (https://github.com/nicobailon/pi-annotate, MIT, by Nico Bailon) has a thorough accessibility capture implementation that extracts:

• ARIA role (explicit or implicit)
• Accessible name and description
• Focusable and disabled state
• ARIA states (expanded, pressed, checked, selected)

This maps directly to M3 (WCAG violation detection). Rather than building from scratch, adapt the pattern from pi-annotate and extend it with violation detection (color contrast, missing alt text, focus order).

Implementation:

• Create src/content/accessibility-inspector.ts following pi-annotate getAccessibilityInfo() pattern
• Run on element selection alongside extractElementData()
• Add to annotation context (supports Include computed styles and DOM subtree in annotation output #21)
• Extend with WCAG violation checks for M3

Attribution: Credit pi-annotate by Nico Bailon in code comments where patterns are adapted.

See docs/design/library-research.md for full analysis.