feat: add responsive ultrawide work lanes

docs/ARCHITECTURE.md

@@ -71,21 +71,23 @@ shutdown against ordered raw-byte output. The GPUI slice feeds one actor session
 through the terminal engine while five surfaces retain deterministic comparison
 output. Its adapter groups adjacent cells into native GPUI paint runs and renders
 ANSI/indexed/RGB colors, emphasis, hyperlinks, and cursor shapes. Window geometry
-drives terminal rows and columns and resizes both the engine and PTY. Selection,
-scrollback navigation, mouse reporting, and a lower-level terminal canvas remain.
-This actor still runs in-process; moving the same framework-neutral
-contract behind local authenticated IPC is the next durability step. See
-decisions 0004 and 0005.
+drives terminal rows and columns and resizes both the engine and PTY. The engine
+now exposes retained-history offsets and page/top/bottom viewport movement
+without writing scroll keys to the PTY. Text selection, mouse reporting, and a
+lower-level terminal canvas remain. This actor still runs in-process; moving the
+same framework-neutral contract behind local authenticated IPC is the next
+durability step. See decisions 0004 and 0005.
 
 ## Default workspace composition
 
-The default desktop workspace is a vertical 3×1 composition: the top 20% holds
-pane, agent, tool, context, and goal state; the middle 60% is the active terminal
-or work surface; the bottom 20% is the answer, choice, chat, and approval shelf.
-Six deterministic surfaces still update in the spike so performance comparisons
-remain meaningful, but only the selected surface owns the large work region.
-Users may deliberately split that region later; a dashboard grid is not the
-calm default. See decision 0008.
+The default desktop workspace has three horizontal bands: the top 20% holds pane,
+agent, tool, context, and goal state; the middle 60% holds responsive vertical
+work lanes; and the bottom 20% is the answer, choice, chat, and approval shelf.
+The work region shows one lane in compact windows, three at normal desktop
+widths, and five on a 3440 px ultrawide. The selected lane alone owns keyboard
+input. Six surfaces continue updating so performance comparisons remain
+meaningful, with a sliding visible window keeping the selected pane onscreen.
+See decisions 0008 and 0009.
 
 We should evaluate, not blindly copy, WezTerm, Zellij, RMUX, tmux, and cmux. The
 first spike must compare a reusable terminal crate with a small first-party layer.