Fix PDF notebook stability and selection gestures

[markm39]

Summary

• Window PDF page backgrounds around the visible page and cache CGPDF documents to reduce large-notebook memory pressure.
• Fix PDF export page selection/render sizing by using requested page indices and consistent CGPDF rendering.
• Route native selection events correctly and block single-finger viewport pan over selected native objects so finger dragging reaches the selected figure/shape.
• Avoid native capture during slot teardown to prevent back/navigation crashes, and preserve imported PDF page counts without adding a disposable blank.

Verification

• npm run typecheck
• npm test
• npm run test:native:smoke
• npm run build
• npm run pack:dry-run
• npm run test:example:typecheck
• npm run test:example:export:ios
• npm run test:example:export:android
• simplify review: no findings
• security review: no findings

Notes

• Replayed over origin/main after Add crisp zoom rendering #33 (feat(ink): add crisp zoom rendering).
• Did not run a device/Xcode build in this session.