Never make the client wait for drawing, or hold up its steps

The renderer process now takes each view's chunks on a thread of its own as
they come, and draws the newest frame on its main thread: the client no longer
waits for the previous frame to be drawn (on a VM, 22 of a frame's 25 ms).

The adapter records the client's steps (SceneViewer::step, by replacing its
entry in the exported IFrameListener vtable, after checking it), and the frame
pacer doesn't ask for a frame that wouldn't end before the next step is due: a
late step made the client catch up two at once, which showed as hitches. While
the renderer process covers the view, the client's renderer doesn't begin or
present frames, which could wait for the display.

The engine's stats now include the time in the client renderer's methods.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

adapter/engine.hpp

@@ -93,6 +93,8 @@ private:
     LARGE_INTEGER frameStart_{}; // when the current frame began
     double frameSeconds_ = 0;    // the stats' window frames' time, beginRendering to endRendering
     bool windowFrame_ = false;   // the current frame is drawn into the window
+    bool innerFrame_ = false;    // the current frame is begun in the client's renderer
+    std::int64_t innerTicksAtStats_ = 0; // innerTicks when the stats' 300 frames started
     int statsLogged_ = 0;
 
 };