[zzet]

Summary

Wait for the first prefix-collision worker to retire before the fixture deletes its owner and retries. The worker sends its result before deferred cleanup clears the in-flight marker, so the old test could race that cleanup and receive an accepted response instead of exercising missing-owner handling.

Closes #1170.

Changes

• Wait for retirement of the exact first worker before changing the owner fixture.
• Preserve both error and guidance assertions, and include returned content in failure messages.

Testing

• Collision control passed under race detection ×20; five relevant tracking/deadline/worktree controls also passed, with no skips.
• Pinned Go 1.27.3 with required CGO flags, golangci-lint 2.14.0 (zero issues), affected-package vet, formatting/diff, CGO/workflow guards, unchanged tidy and module verification passed at a1eba0c47.
• This changes one test file. Performance benchmarks and extractor metadata/edges do not apply.

Checklist

• Relevant automated checks passed before pushing.
• Existing fixture and worker-lifecycle patterns retained.
• No new abstraction or production behavior change.