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>

package.json

@@ -8,6 +8,8 @@
     "build": "vite build",
     "preview": "vite preview",
     "check": "svelte-check --tsconfig ./tsconfig.json",
+    "test": "vitest run",
+    "test:watch": "vitest",
     "tauri": "tauri"
   },
   "dependencies": {
@@ -22,6 +24,7 @@
     "svelte": "^5.0.0",
     "svelte-check": "^4.0.0",
     "typescript": "^5.6.0",
-    "vite": "^5.4.0"
+    "vite": "^5.4.0",
+    "vitest": "^2.0.0"
   }
 }