feat: add responsive ultrawide work lanes

docs/UX_VERTICAL_SLICE.md

@@ -10,12 +10,14 @@ framework decision gate, not a decorative dashboard.
 ## Current-run audit
 
 The first 2026-08-31 GPUI baseline proved six-surface density but gave every
-pane equal visual priority. The current slice replaces that dashboard with a
-3×1 default: 20% context and pane selection, 60% active work surface, and 20%
-decision shelf. Keyboard focus, command-palette input, live PTY input, styled VT
-cells, cursor shapes, and geometry-driven resize are wired. Remaining visible
-gaps are terminal selection/scrollback, real decision-shelf actions, usage-detail
-provenance, and end-to-end platform accessibility/IME.
+pane equal visual priority. The current slice replaces that dashboard with three
+horizontal bands: 20% context and pane selection, a 60% responsive work region,
+and a 20% decision shelf. The work region shows one, three, or five vertical
+lanes according to available width. Keyboard focus, command-palette input, live
+PTY input, styled VT cells, cursor shapes, geometry-driven resize, and retained-
+history navigation are wired. Remaining visible gaps are terminal text selection
+and mouse modes, real decision-shelf actions, usage-detail provenance, and end-to-
+end platform accessibility/IME.
 
 Screenshots for the audit are stored outside Git under
 `~/shots/2026-08/lumbridge-ui-audit/`. Accessibility and IME correctness cannot
@@ -40,8 +42,8 @@ research, not source assets for Lumbridge. We adapt these interaction patterns:
   cards;
 - a quiet top tab strip for mixed terminal, Markdown, browser, and review
   surfaces;
-- one primary work surface with optional splits, instead of forcing every
-  surface into an equal dashboard tile;
+- a primary work region with a small responsive number of equal vertical lanes,
+  instead of either one ultrawide terminal or a dense equal-card dashboard;
 - narrow contextual tools, such as files, review, or Buzz, that can collapse
   when the terminal needs the space;
 - usage and agent state at the edge of the workspace rather than in modal
@@ -62,8 +64,9 @@ surface rather than a hosted Lumbridge control plane.
    the selected pane receives the accent treatment.
 4. A waiting pane uses an amber semantic label in both the context band and
    attention sidebar. The label always includes words such as `NEEDS INPUT`.
-5. The middle 60% gives the selected terminal or work surface uninterrupted
-   reading space and retains target, harness state, dimensions, and surface tabs.
+5. The middle 60% gives one, three, or five terminal/work lanes uninterrupted
+   height and retains target, harness state, dimensions, and surface tabs. The
+   selected lane alone owns keyboard input.
 6. The footer groups connection state, selected-harness identity, usage-window
    provenance, and burn forecast into readable regions.
 7. The selected pane has a quiet surface strip for Terminal, Browser, Tools,
@@ -79,6 +82,8 @@ surface rather than a hosted Lumbridge control plane.
 - `Alt+1` through `Alt+6`: focus a pane directly while leaving terminal digits
   available to the PTY.
 - `Cmd+K` on macOS or `Ctrl+K` on Linux: open the command palette.
+- `Shift+PageUp/PageDown`: move the selected live terminal through retained
+  history; `Shift+Home/End` jumps to the history top/live bottom.
 - Typing while the palette is open changes its query; arrows change the result;
   `Enter` runs it; `Escape` closes it and restores pane focus.
 - `Enter` on a waiting agent opens its request detail. Approval remains a
@@ -92,8 +97,9 @@ receive the same state transitions and tests.
 
 - In the first actor integration, one terminal pane consumes ordered output from
   a real local PTY while each deterministic tick updates the other five
-  surfaces offscreen. The all-deterministic constructor remains available for
-  framework comparison and replay tests.
+  surfaces. One, three, or five are visible according to available width; the
+  all-deterministic constructor remains available for framework comparison and
+  replay tests.
 - One pane enters and leaves `needs input` through a deterministic event.
 - Markdown, browser-boundary, and review panes update counters without using a
   web application shell.