docs: clarify continuous canvas positioning

[markm39]

Summary

• add README positioning that explains Mobile Ink's continuous notebook canvas wedge
• compare PencilKit, React Native Skia, web canvas libraries, custom native modules, and Mobile Ink
• add matching npm discovery keywords for React Native Skia, Expo, Android, handwriting canvas, Apple Pencil support, and native modules

Verification

• Simplify review: no findings
• Security review: no findings
• node -e "JSON.parse(require('fs').readFileSync('package.json','utf8')); console.log('package.json ok')"
• git diff --check
• npm pack --dry-run --ignore-scripts