ref(config): Switch snapshot test pool from forks to threads

Use threads pool instead of forks for snapshot tests while keeping
single-worker concurrency via maxThreads: 1. Threads have lower
overhead than forked processes for these serialized test runs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: cameroncooke

vitest.snapshot.config.ts

@@ -5,10 +5,10 @@ export default defineConfig({
     environment: 'node',
     globals: true,
     include: ['src/snapshot-tests/__tests__/**/*.test.ts'],
-    pool: 'forks',
+    pool: 'threads',
     poolOptions: {
-      forks: {
-        maxForks: 1,
+      threads: {
+        maxThreads: 1,
       },
     },
     env: {