ui::bus / Tier C send_message/process_message endpoints over moodycamel
queues: UI thread enqueues (may allocate), audio thread try_enqueues on
preallocated storage and never blocks. avnd::bus_serial handles
non-trivially-copyable payloads (string/vector/aggregate); payloads a
serializer can't handle silently disable that bus direction, by design.

Co-Authored-By: Claude Fable 5 <[email protected]>

Editors resolve through avnd::make_ui_editor (author-provided Tier C
window, or the reference editor when a soft-UI backend is enabled at
build time). Automation gestures flow editor -> lock-free gesture queue
-> host params request_flush -> CLAP_EVENT_PARAM_GESTURE_BEGIN /
PARAM_VALUE / GESTURE_END in params.flush. The message bus rides the
same lock-free transport, drained in process() on the audio side and on
the editor timer on the UI side.

Co-Authored-By: Claude Fable 5 <[email protected]>

Same editor resolution and bus transport as the CLAP binding;
automation gestures map to audioMaster BeginEdit/Automate/EndEdit and
processor->UI messages drain on EditIdle and the editor frame hook.

Co-Authored-By: Claude Fable 5 <[email protected]>

- plug_view drives an avnd::gui_windowed_ui editor: controller-side
  model instance, IComponentHandler begin/perform/endEdit gestures,
  IPlugViewContentScaleSupport, and a UI-thread tick (Win32 SetTimer /
  host IRunLoop on Linux / CFRunLoopTimer on macOS) that runs
  editor->idle() and the bus pump.
- The message bus crosses the component/controller split via IMessage
  with the pfr serializer; IMessage is only allocated/sent on the UI
  thread (see the bus.hpp header comment for the pump protocol), the
  audio thread only enqueues.
- The prototype defines the plug-view + Linux windowing IIDs
  (IRunLoop, ITimerHandler, IEventHandler): an unresolved reference to
  any of them would pull commoniids.cpp.o out of libsdk_common.a, which
  then collides with every DEF_CLASS_IID under GNU ld.

Co-Authored-By: Claude Fable 5 <[email protected]>

CustomUiWindow: a gain with a hand-rolled editor (raw Win32 + GDI on
Windows, raw Xlib on Linux) through the avnd::gui_windowed_ui seam — no
UI library at all, so the contract is the whole story. Tested through
the CLAP binding by tests/ui/test_custom_ui_window.cpp: embed in a real
parent window, injected drag, gesture flow, message-bus round trip.
tests/ui/gui_test_host.hpp is the shared Win32/X11 host layer (parent
window, event pump with per-tick host callback, child discovery,
synthetic input, PPM capture) that the soft-editor tests reuse later.

Co-Authored-By: Claude Fable 5 <[email protected]>

The end-to-end editor tests embed real windows; without X11 dev headers
they are skipped at configure time, and without a display they cannot
run. Install libx11/xrandr/xext/xcursor + xvfb and run the test steps
under xvfb-run on Linux.

Co-Authored-By: Claude Fable 5 <[email protected]>