Verify updater signatures before replacing the app

[justingleader]

Summary

• require downloaded update bundles to match SpeakType's expected bundle ID and developer team
• validate the mounted app's code signature and Gatekeeper acceptance before replacing the running app
• add focused unit tests for the new signing checks

Testing

• xcodebuild -project /Users/justinleader/Documents/Codex/2026-04-23-examine-this-fully-and-see-if/speaktype/speaktype.xcodeproj -scheme speaktype build-for-testing -destination 'platform=macOS' MACOSX_DEPLOYMENT_TARGET=15.0 CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO -only-testing:speaktypeTests/UpdateServiceSecurityTests\n- xcodebuild -project /Users/justinleader/Documents/Codex/2026-04-23-examine-this-fully-and-see-if/speaktype/speaktype.xcodeproj -scheme speaktype test-without-building -destination 'platform=macOS' MACOSX_DEPLOYMENT_TARGET=15.0 CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO -only-testing:speaktypeTests/UpdateServiceSecurityTests