What changed

• Ported the tvOS download implementation from https://github.com/Lakr233/ApplePackage.
• Added platform parsing and platform-aware search/lookup entities for iphone, ipad, and appletv.
• Added tvOS download support by resolving the platform-specific external version id through Apple's MDM lockup endpoint before the store download request.
• Added AppleTVOS package validation after patching so a tvOS download fails visibly when the package metadata does not declare tvOS support.
• Documented the new --platform flag for search and download.

Issues

Closes #46
Related #27

Validation

• go generate ./...
• go test ./...
• go build ./...
• Real account test using local ~/.ipatool credentials: refreshed auth token, searched Infuse with --platform appletv, downloaded com.firecore.infuse to /tmp/ipatool-tvos-test/infuse-appletv.ipa, and verified Payload/infuse.app/Info.plist declares CFBundleSupportedPlatforms = AppleTVOS.

No credentials, cookies, account files, or downloaded IPA files are included in this PR.