Add bounded runtime actor and live PTY pane

docs/UI_SPIKE_SCORECARD.md

@@ -83,10 +83,11 @@ fallback.
 
 ## Measurement semantics
 
-Both renderers consume the same deterministic six-surface action stream. Each
-tick invalidates six surfaces, appends one bounded line to each of the three
-terminal fixtures, and records stable event/revision counters. The GPUI footer
-currently reports dispatch-to-element-build p50/p95 over a bounded 256-sample
-window. It is deliberately not called key-to-present or frame-present latency:
-neither candidate exposes a reliable public cross-platform post-present
-callback. True present latency requires an external platform probe.
+Both renderers retain the same all-deterministic six-surface action stream for
+comparison. The GPUI integration mode replaces one terminal fixture with a real
+actor-owned PTY and leaves five deterministic surfaces running. Counters
+separate external PTY batches/lines from total model updates. The GPUI footer
+reports dispatch-to-element-build p50/p95 over a bounded 256-sample window. It
+is deliberately not called key-to-present or frame-present latency: neither
+candidate exposes a reliable public cross-platform post-present callback. True
+present latency requires an external platform probe.