feat: add responsive ultrawide work lanes

docs/UI_SPIKE_SCORECARD.md

@@ -24,9 +24,9 @@ from macOS and Linux and the hard gates pass.
 The current programs establish dependency, build, launch, and interaction
 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.
+from its responsive one/three/five-lane 60% terminal band when the window
+changes. Retained-history page/top/bottom navigation is wired. Text selection,
+mouse modes, a native Markdown editor, and one isolated browser child remain.
 
 ## Ubuntu baseline — metal, 2026-08-31
 
@@ -87,9 +87,9 @@ fallback.
 ## Measurement semantics
 
 Both renderers retain the same all-deterministic six-surface action stream for
-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
+comparison. GPUI now presents a 20/60/20 workspace whose middle band shows one,
+three, or five vertical work lanes while one terminal fixture is replaced with a
+real actor-owned VT session and five deterministic surfaces keep 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