Summary

• add Sparkle 2 as the macOS update framework
• add a native Check for Updates menu item
• move app metadata into a real Info.plist with Sparkle feed/public key settings
• add an appcast generation script and Sparkle update docs

Verification

• swift test
• xcodebuild -project SlamDih.xcodeproj -scheme SlamDih -configuration Debug -destination 'platform=macOS' CODE_SIGNING_ALLOWED=NO build
• ./scripts/create-dmg.sh 0.1.0
• ./scripts/generate-appcast.sh .build/dmg
• verified appcast includes sparkle:edSignature