fix(plugins): stabilize VST3 and AU editor lifecycle, thread affinity, and scanning

[Horuse]

What does this PR do?

Fixes crashes, black/blank screens, and lifecycle issues when scanning, opening, and interacting with VST3 and AU plugin GUI editors (notably JUCE and complex UI plugins like Tokyo Dawn Labs TDR Nova and Native Instruments Replika):

• Strict Main Thread affinity for VST3 and AU: Dispatches all plugin-related commands (open_plugin_editor, close_plugin_editor, get_plugin_state, plugin_status, get_plugin_params), bundle scanning, and host activations (Vst3Host::activate, AuHost::activate) strictly on the Main Thread. This prevents Tokio and audio engine background threads from triggering data races or AppKit/COM violations when initializing plugins (fixes startup crashes with AU plugins like Native Instruments Replika and scanner crashes on background threads).
• Flexible editor sizing (getSize lifecycle): Ensures IPlugView::setFrame is called before sizing and queries getSize() both before and after attached(), since many plugins (e.g. Native Instruments) only report layout dimensions after native window attachment. Additionally inspects the attached native subview frame on macOS and provides a fallback size (800x600) rather than failing with editor reported no size.
• Hide/Show window lifecycle: Toggling a plugin window now hides and shows the native window (w.hide() / w.show()) instead of destroying and recreating the IPlugView COM object each time. This prevents COM teardown churn and heap corruption in plugins that rely on persistent static UI state.
• macOS compositing & layout: Enables setWantsLayer: true on the parent view, cleans up stale subviews, and notifies the view via onFocus(1) and setNeedsDisplay. Removes artificial coordinate insetting that previously corrupted the OpenGL/Metal viewport.
• Pipeline reconcile re-attaching: When connecting cables or changing pipeline configuration, the new audio instance automatically re-attaches its editor view to the existing window so the UI does not go blank.
• Clean window teardown on plugin swap: Switching to a different plugin or format on a node now explicitly closes the old window so the new plugin opens fresh.
• Cached has_editor: Caches the editor check in Vst3Instance to avoid repeated createView(kEditor) calls during periodic status polling.

Fixes #34
Fixes #35

Why is this the right approach?

• The crashes were caused by thread-affinity violations (calling COM methods or Cocoa/AU initialization from Tokio or engine threads) and repeated IPlugView destruction, not by WebView2 or message loop collisions.
• The hide/show model is the standard approach used by production DAWs (Reaper, Bitwig, etc.) to keep complex plugin editors stable.
• The solution is completely in-process, adds zero audio latency, introduces no new dependencies, and fixes the issues across both macOS and Windows.
• It touches no real-time audio threads (cpal / SCK callbacks or DspWorker::run remain completely wait-free and untouched).

How was it tested?

• macOS (Sonoma, ARM64):
  • Scanned and loaded complex VST3/AU plugins:
    • Tokyo Dawn Labs TDR Nova (VST3): Verified OpenGL/Metal GUI renders correctly without black screens.
    • Native Instruments Replika (VST3 & AU): Verified that scanning no longer crashes (confirming [crash] Crash when scanning Native Instruments plugins for plugin node #34 is resolved), AU instantiation no longer crashes on pipeline startup/reconcile, and VST3 editor opens and attaches without failing on editor reported no size.
  • Tested connecting new nodes/cables (pipeline reconcile) while an editor is open; verified GUI stays responsive and re-attaches cleanly without going blank.
  • Verified switching plugins on a node closes the previous window and opens the new plugin cleanly.
  • Ran cargo test --lib (all 139 tests passed).
• Windows / Linux:
  • Not tested on Linux or Windows: The specific proprietary plugins causing these crashes (TDR Nova, NI Replika) are unavailable for ARM64 Linux, and testing hardware was macOS-only.
  • The thread-safety fixes use cross-platform main-thread dispatch (main_thread::run using thread ID validation) and standard OS window parenting, ready for testing on Windows by contributors with Windows machines.

Checklist

• Read docs/CONTRIBUTING.md and docs/RULES.md.
• Kept the diff to the change. No drive-by renames, reformatting, or
  refactors of code the change does not touch.
• Tested on every OS this change affects (macOS fully verified; Windows/Linux not tested due to ARM64 and hardware constraints, see notes above).
• Checked that bun run check passes.
• Checked that cargo check --manifest-path src-tauri/Cargo.toml passes.
• Checked that bun run format leaves the tree clean.
• If changing a #[derive(TS)] type, ran bun run generate and
  committed the regenerated files.