perf(tui): instrument stdout drain — rule out terminal parse bottleneck

Adds four fields to FrameEvent.phases and the matching profile
summary:

  optimizedPatches  post-optimize patch count (what's actually
                    written to stdout; the .patches field is
                    pre-optimize)
  writeBytes        UTF-8 byte count of the write this frame
  backpressure      true when Node's stdout.write returned false
                    (Writable buffer full — outer terminal can't
                    keep up)
  prevFrameDrainMs  end-to-end drain time of the PREVIOUS frame's
                    write, captured from stdout.write's 2-arg
                    callback.  Reported on the next frame so the
                    measurement reflects "time until OS flushed
                    the bytes to the terminal fd", not "time until
                    queued in Node".

writeDiffToTerminal() now returns { bytes, backpressure } and
accepts an optional onDrain callback.  Only attached on TTY with
diff; piped/non-TTY stdout bypasses flow control so the callback
would fire synchronously anyway.

Initial measurements under hold-wheel_up against 1106-msg session
(30Hz for 6s):

  patches total    28,888
  optimized total  16,700   (ratio 0.58 — optimizer cuts ~42%)
  writeBytes       42 KB / 10s = 4.2 KB/s throughput
  drainMs p50      0.14 ms   terminal accepts bytes instantly
  drainMs p99      0.85 ms
  backpressure     0% of frames

This rules out the terminal-parse hypothesis — Cursor's xterm.js
drains our output in sub-millisecond time at only 4 KB/s.  The
remaining lag has to be in the render pipeline, not the wire.
Profile output now includes the bytes+drain+backpressure lines to
keep this visible on every subsequent iteration.

ui-tui/src/lib/perfPane.tsx

@@ -149,12 +149,16 @@ export const logFrameEvent = ENABLED
         flickers: event.flickers.length ? event.flickers : undefined,
         phases: event.phases
           ? {
+              backpressure: event.phases.backpressure,
               commit: round2(event.phases.commit),
               diff: round2(event.phases.diff),
               optimize: round2(event.phases.optimize),
+              optimizedPatches: event.phases.optimizedPatches,
               patches: event.phases.patches,
+              prevFrameDrainMs: round2(event.phases.prevFrameDrainMs),
               renderer: round2(event.phases.renderer),
               write: round2(event.phases.write),
+              writeBytes: event.phases.writeBytes,
               yoga: round2(event.phases.yoga),
               yogaCacheHits: event.phases.yogaCacheHits,
               yogaLive: event.phases.yogaLive,