fix(kernel-utils): restore definite-assignment assertion for releaseLock

The build tsconfig (with strict mode) requires the `!` assertion on
`releaseLock` since TypeScript cannot verify that the Promise constructor
callback runs synchronously.

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

Author: sirtimid

packages/kernel-utils/src/vite-plugins/bundle-vat.ts

@@ -74,7 +74,7 @@ export async function bundleVat(sourcePath: string): Promise<VatBundle> {
   // Wait for any in-flight build to finish before starting ours, then
   // register a slot so the next caller waits for us.
   const prevQueue = buildQueue;
-  let releaseLock: () => void;
+  let releaseLock!: () => void;
   buildQueue = new Promise<void>((resolve) => {
     releaseLock = resolve;
   });