Pace all frames to the frame rate, at best effort

The pacer counted only its own frames, so the client's 30 came on top (60 gave
about 80), and it added no frames at all when the rate was out of reach. It now
asks for a window's next frame once the interval since its last frame began has
passed, and the client has had a third of a frame's time idle: all frames
within the rate, and as many as the client can draw when it can't reach it.

The engine's stats also break a frame's time down: sending to the renderer
process, bytes sent, buffer updates.

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

adapter/engine.hpp

@@ -91,6 +91,7 @@ private:
     Stats stats_;
     LARGE_INTEGER statsStart_{}; // when the stats' 300 frames started
     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
     int statsLogged_ = 0;