vst3-host: host plugin editors on Windows

Adds a Windows implementation of the editor host window: a top-level HWND
owned by the app's active window (no taskbar button), sized from the
view's rect, resized on IPlugFrame::resizeView, hidden on close and torn
down by the editor link's idle like on macOS. The main thread is the one
Rust names "main", where the app runs its message loop.

Also fixes two Windows-only build problems that kept the VST3 tests from
running there: the fixture's bus flags (the vst3 bindings use i32 enums
with MSVC) and an unused variable in a unix-only test.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.lock

@@ -3640,6 +3640,7 @@ dependencies = [
  "soundcraft-dsp",
  "thiserror 2.0.21",
  "vst3",
+ "windows-sys 0.61.2",
 ]
 
 [[package]]