feat: add responsive ultrawide work lanes

docs/TESTING.md

@@ -118,10 +118,14 @@ who already have the final cargo-watch release installed.
   blocked OSC 52 behavior, key encoding, application-cursor mode, and resize.
 - `lumbridge-core` tests capability-gated split/select/rename/close commands,
   idempotent request IDs, close-tree promotion, and atomic agentic setup plans.
-- The GPUI slice tests its key-event adapter, 20/60/20 geometry-to-PTY sizing,
-  xterm 256-color conversion, styled-run coalescing, and cursor-run boundaries.
-  It renders one real actor-owned VT session while five surfaces continue their
-  deterministic background workload.
+- The GPUI slice tests its key-event adapter, responsive one/three/five-lane
+  geometry, selected-pane lane windowing, 20/60/20 geometry-to-PTY sizing, xterm
+  256-color conversion, styled-run coalescing, cursor-run boundaries, and
+  scrollback shortcut routing. It renders one real actor-owned VT session while
+  five surfaces continue their deterministic background workload.
+- `lumbridge-terminal` retains 10,000 history lines by default and tests
+  framework-neutral page/top/bottom viewport movement, revision changes, and
+  live-bottom no-ops without sending history-navigation bytes to the child PTY.
 - The current-GPUI probe compile-checks real AccessKit element wiring and real
   platform input-handler installation. Unit tests cover its semantic tree and
   UTF-16/UTF-8 composed-text mutations. OS screen readers, IME candidate windows,