Add M2 splits-tree layout

- src/lib/layout/tree.ts: pure helpers + types (newLeaf, splitLeaf,
  closeLeaf, replaceById, serialize/deserialize with shape-checking).
- SplitNode.svelte: flex container with pointer-captured gutter drag.
- LeafPane.svelte: per-pane toolbar (split-right ⇥, split-down ⇣,
  close ×) over the existing XtermPane.
- Pane.svelte: recursive dispatcher between SplitNode and LeafPane,
  keyed on leaf.id so swaps unmount XtermPane cleanly (kills PTY).
- App.svelte: tree-as-state with split/close handlers, auto-save to
  localStorage on every \$effect tick. Titlebar shows clickable distro
  buttons setting the default for new panes; existing panes keep theirs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/layout/Pane.svelte

@@ -0,0 +1,23 @@
+<script lang="ts">
+  import type { TreeNode, NodeId, Orientation } from "./tree";
+  import SplitNode from "./SplitNode.svelte";
+  import LeafPane from "./LeafPane.svelte";
+
+  let {
+    node,
+    onSplit,
+    onClose,
+  }: {
+    node: TreeNode;
+    onSplit: (leafId: NodeId, orientation: Orientation) => void;
+    onClose: (leafId: NodeId) => void;
+  } = $props();
+</script>
+
+{#if node.kind === "split"}
+  <SplitNode {node} {onSplit} {onClose} />
+{:else}
+  {#key node.id}
+    <LeafPane leaf={node} {onSplit} {onClose} />
+  {/key}
+{/if}