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-tauri/Cargo.lock

@@ -3436,6 +3436,24 @@ dependencies = [
  "cfg-if",
 ]
 
+[[package]]
+name = "tiletopia"
+version = "0.0.1"
+dependencies = [
+ "anyhow",
+ "base64 0.22.1",
+ "once_cell",
+ "parking_lot",
+ "portable-pty",
+ "serde",
+ "serde_json",
+ "tauri",
+ "tauri-build",
+ "tokio",
+ "tracing",
+ "tracing-subscriber",
+]
+
 [[package]]
 name = "time"
 version = "0.3.47"
@@ -4730,24 +4748,6 @@ dependencies = [
  "x11-dl",
 ]
 
-[[package]]
-name = "wsl-mux-spike"
-version = "0.0.1"
-dependencies = [
- "anyhow",
- "base64 0.22.1",
- "once_cell",
- "parking_lot",
- "portable-pty",
- "serde",
- "serde_json",
- "tauri",
- "tauri-build",
- "tokio",
- "tracing",
- "tracing-subscriber",
-]
-
 [[package]]
 name = "x11"
 version = "2.21.0"