test(project): stabilize VCS branch update test

[kitlangton]

Summary

• Register the VCS branch-update test subscription synchronously with Bus.subscribeCallback
• Retry synthetic HEAD update events until the branch update is observed, avoiding missed-event races

Test Plan

• bun run test test/project/vcs.test.ts
• bun typecheck
• for i in {1..25}; do bun run test test/project/vcs.test.ts >/tmp/vcs-test-$i.log 2>&1 || { cat /tmp/vcs-test-$i.log; exit 1; }; done