Independent custom windows (M6)

et.custom-window.v1 is an optional C service. Declare the custom-window capability and feature in the plugin manifest, then request ET_SERVICE_CUSTOM_WINDOW or use et::sdk::Window. The service registers one worker-owned window per plugin. show() opens or focuses it; focus() and close() require no broker UI model. A user close is reported through is_open after the next tick(). The worker calls tick() at approximately 60 Hz.

All calls and callbacks are serialized on the worker dispatcher. Native window, graphics and ImGui pointers stay in the plugin module; no pointer crosses IPC. Callbacks should return promptly so heartbeats and ET service results can run. Exceptions or failed frames fail only that plugin worker. Worker shutdown closes the window before the plugin's stop() callback; stop() should also clean up idempotently. Background jobs cannot invoke the window service.

The imgui-inspector example builds against the exported SDK. It uses GLFW 3.4 and Dear ImGui 1.92.3 with OpenGL 3.3 on Windows and Linux X11. Both source distributions and their licenses are included under vendor/. On Linux, install OpenGL and X11 development packages including Xrandr, Xinerama, Xcursor and Xi. The example has no Qt dependency.

Open Examples / ImGui inspector in the Plugins menu, then use Open / reopen. The ET panel offers focus and close; the independent window plots live RPM and can select another channel or refresh ET metadata. The native window uses GLFW input and updates ImGui font scale from the current monitor content scale. This is an independent OS window, not an embedded or docked Qt Quick surface.

Windows acceptance: scripts/verify-plugin-window.ps1 builds the exported example, verifies actual nonuniform OpenGL framebuffer pixels, native mouse and keyboard events, resize, close/reopen, worker crash containment and ET panel registration. It can run with a system-only PATH against a staged host. Linux uses the same source/backend with X11. Pi/ARM64 rendering, a physical second monitor, monitor hot-plug and Wayland are separate platform gates.