fix(terminal): keep host-retired panes out of every layout a paired client writes (#33083)

Exiting a paired host tab's original pane moved the host's active leaf to the
surviving split. The client's reconciliation effect followed it with
setActivePane, and that persisted every still-mounted pane, writing the retired
leaf (whose remote PTY had not ended yet) back into the layout. The leaf was then
no longer retired, so its pane stayed forever. Any other persist in that window
(insertions, clicks, title edits, divider drags) could do the same.

persistLayoutSnapshot now drops the leaves the host retired, using the
retireLeavesFromTerminalLayout main already uses, moved to shared. The retired
set it reads is the one the reconciliation effect tracks.

src/shared/terminal-layout-leaf-retirement.ts

@@ -0,0 +1,63 @@
+import type { TerminalLayoutSnapshot, TerminalPaneLayoutNode } from './terminal-tab-types'
+import { collectLeafIds } from './terminal-pane-layout-tree'
+
+function pruneTerminalPane(
+  node: TerminalPaneLayoutNode | null,
+  retiredLeafIds: ReadonlySet<string>
+): TerminalPaneLayoutNode | null {
+  if (!node) {
+    return null
+  }
+  if (node.type === 'leaf') {
+    return retiredLeafIds.has(node.leafId) ? null : node
+  }
+  const first = pruneTerminalPane(node.first, retiredLeafIds)
+  const second = pruneTerminalPane(node.second, retiredLeafIds)
+  if (first && second) {
+    return { ...node, first, second }
+  }
+  return first ?? second
+}
+
+function omitLeafRecords<T>(
+  values: Record<string, T> | undefined,
+  retiredLeafIds: ReadonlySet<string>
+): Record<string, T> | undefined {
+  if (!values) {
+    return undefined
+  }
+  const retained = Object.fromEntries(
+    Object.entries(values).filter(([leafId]) => !retiredLeafIds.has(leafId))
+  )
+  return Object.keys(retained).length > 0 ? retained : undefined
+}
+
+/** Drops leaves and their per-leaf records; null when no leaf remains. */
+export function retireLeavesFromTerminalLayout(
+  layout: TerminalLayoutSnapshot,
+  retiredLeafIds: ReadonlySet<string>
+): TerminalLayoutSnapshot | null {
+  const root = pruneTerminalPane(layout.root, retiredLeafIds)
+  if (!root) {
+    return null
+  }
+  const retainedLeafIds = collectLeafIds(root)
+  const retainedLeafIdSet = new Set(retainedLeafIds)
+  const activeLeafId =
+    layout.activeLeafId && retainedLeafIdSet.has(layout.activeLeafId)
+      ? layout.activeLeafId
+      : retainedLeafIds[0]!
+  return {
+    ...layout,
+    root,
+    activeLeafId,
+    expandedLeafId:
+      layout.expandedLeafId && retainedLeafIdSet.has(layout.expandedLeafId)
+        ? layout.expandedLeafId
+        : null,
+    ptyIdsByLeafId: omitLeafRecords(layout.ptyIdsByLeafId, retiredLeafIds),
+    buffersByLeafId: omitLeafRecords(layout.buffersByLeafId, retiredLeafIds),
+    scrollbackRefsByLeafId: omitLeafRecords(layout.scrollbackRefsByLeafId, retiredLeafIds),
+    titlesByLeafId: omitLeafRecords(layout.titlesByLeafId, retiredLeafIds)
+  }
+}