feat: make terminal the primary workspace surface

docs/UI_SPIKE_SCORECARD.md

@@ -22,10 +22,11 @@ from macOS and Linux and the hard gates pass.
 | API clarity and maintenance burden | review | pending | pending |
 
 The current programs establish dependency, build, launch, and interaction
-baselines. GPUI now routes a real PTY through a VT engine and keyboard encoder;
-its renderer still flattens the immutable screen snapshot to plain rows. The
-next iteration adds styled cells/cursor/selection, measured pane-driven resize,
-a native Markdown editor, and one isolated browser child.
+baselines. GPUI now routes a real PTY through a VT engine and keyboard encoder,
+paints coalesced styled cell runs and cursor shapes, and derives PTY rows/columns
+from its 60% terminal band when the window changes. Selection, scrollback
+navigation, mouse modes, a native Markdown editor, and one isolated browser child
+remain.
 
 ## Ubuntu baseline — metal, 2026-08-31
 
@@ -86,8 +87,9 @@ fallback.
 ## Measurement semantics
 
 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 VT session and leaves five deterministic surfaces running. Counters
+comparison. GPUI now presents the selected surface in a 20/60/20 vertical
+workspace while one terminal fixture is replaced with a real actor-owned VT
+session and five deterministic surfaces keep running offscreen. 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