Add vitest + 43 unit tests for tree.ts

Setup:
- vitest 2.x devDep; pnpm test / pnpm test:watch scripts.
- vite.config.ts test: block (node env, src/**/*.test.ts) via vitest/config.

Coverage in tree.test.ts:
- newLeaf / newSplit (defaults + provided props).
- replaceById (root/nested/no-match, immutability + sibling reuse).
- splitLeaf (orientation, inheritance, no-op on missing id, nested).
- closeLeaf (root -> null, sibling collapse, nested removal, no-op).
- findLeaf / leafCount / walkLeaves (order).
- changeDistro pins the invariant that it MUST swap the leaf id
  ({#key} remounts XtermPane → kills+respawns PTY).
- changeLabel / toggleBroadcast pin the inverse invariant: id MUST
  remain stable (metadata-only mutations).
- All 5 presets: shape, distro propagation, fresh ids per call.
- serialize/deserialize roundtrip + invalid-input rejection.

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

vite.config.ts

@@ -1,3 +1,4 @@
+/// <reference types="vitest" />
 import { defineConfig } from "vite";
 import { svelte } from "@sveltejs/vite-plugin-svelte";
 
@@ -16,4 +17,8 @@ export default defineConfig(async () => ({
     minify: "esbuild",
     sourcemap: false,
   },
+  test: {
+    environment: "node",
+    include: ["src/**/*.test.ts"],
+  },
 }));