fix(tui): center prose measure on shared spine

packages/tui/src/ui/layout.ts

@@ -6,10 +6,13 @@ export function sessionTabsFitVertically(total: number) {
   return total >= SESSION_SIDEBAR_WIDTH + SESSION_CONTENT_MIN_WIDTH
 }
 
+// The shared spine centers the prose measure; the technical rail shares its
+// leading edge and extends rightward, clamped so it still fits the canvas.
 export function sessionLaneLayout(available: number, readable: number) {
   const technical = Math.min(available, Math.max(readable, SESSION_TECHNICAL_LANE_WIDTH))
+  const centered = Math.floor((available - Math.min(readable, technical)) / 2)
   return {
-    inset: Math.max(0, Math.floor((available - technical) / 2)),
+    inset: Math.max(0, Math.min(centered, available - technical)),
     readable: Math.min(readable, technical),
     technical,
   }

packages/tui/test/ui/layout.test.ts

@@ -12,9 +12,12 @@ test("vertical tabs match the session sidebar and preserve compact content width
   expect(sessionTabsFitVertically(85)).toBe(false)
 })
 
-test("session lanes share one leading edge", () => {
+test("session lanes center the prose measure on one leading edge", () => {
   expect(SESSION_TECHNICAL_LANE_WIDTH).toBe(88)
-  expect(sessionLaneLayout(156, 66)).toEqual({ inset: 34, readable: 66, technical: 88 })
+  // Wide canvas: prose is truly centered, technical extends rightward.
+  expect(sessionLaneLayout(156, 66)).toEqual({ inset: 45, readable: 66, technical: 88 })
+  // Centering the prose would push the technical rail past the canvas; clamp.
+  expect(sessionLaneLayout(100, 66)).toEqual({ inset: 12, readable: 66, technical: 88 })
   expect(sessionLaneLayout(80, 66)).toEqual({ inset: 0, readable: 66, technical: 80 })
   expect(sessionLaneLayout(60, 66)).toEqual({ inset: 0, readable: 60, technical: 60 })
 })