feat: expose footnote lifecycle hooks

[rhnvrm]

Summary

• add public entries and refresh() surfaces for source-to-preview mappings and dynamic content
• emit oatbase:render after previews are ready and oatbase:toggle as previews open or close
• expose data-footnote-for on generated previews for stable placement integration
• retain ordinary source links and definitions when Popover is unavailable
• document a script-order-independent sidenote/layout integration and add focused lifecycle coverage

Verification

• source/docs suite: 52 passed
• component and documentation browser smokes passed
• Chromium component suite: 24 passed, including no-JS, dynamic content, axe, keyboard, themes, contrast, and visual baselines
• complete bundle: 27,294 gzip bytes (28,000 budget)
• extensions bundle: 17,635 gzip bytes (18,000 budget)