refactor(vst): structure WebView native events

[thorinf]

Summary

• route native-to-WebView updates through one structured tram8.handleNativeEvent(event) dispatcher
• send full gate state as one JSON event instead of eight string-built JavaScript calls
• keep JS-to-native commands on WKScriptMessageHandler postMessage, matching the current macOS 10.15 target
• harden activity flash delivery by taking a PlugView ref and synchronizing activeView access

Review

• checked Apple WebKit docs: WKScriptMessageHandler is appropriate for fire-and-forget JS-to-native commands; WKScriptMessageHandlerWithReply is not needed because the UI does not need Promise-style replies
• extra-high agent reviewed the diff twice, plus a final synchronization check; no blocking findings remain

Tests

• git diff --check
• make -C vst/tests test
• XCODE_VERSION=15.0 cmake --build vst/build --target tram8-bridge (compiles and Steinberg validator passes 47/47, then fails at the known local VST3 install link delete: ~/Library/Audio/Plug-Ins/VST3/tram8-bridge.vst3: Operation not permitted)