Add runner versioning to RivetKit registry

Uses Date.now() so each process start gets a unique version.
This ensures Rivet Cloud migrates actors to the new runner on
deploy instead of routing requests to stale runners.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

foundry/packages/backend/src/actors/index.ts

@@ -12,6 +12,9 @@ export const registry = setup({
   serverless: {
     basePath: "/v1/rivet",
   },
+  runner: {
+    version: Date.now(),
+  },
   use: {
     workspace,
     project,