Phase 11: Add stress and race tests

[rpmoore]

Roadmap step 93 from docs/steps.md.

Scope

Add stress and race tests.

Details

Stress concurrent DNS queries, cache single-flight behavior, snapshot swaps, slow event writing, blocklist activation, and config reload while queries are in flight.

References

• docs/steps.md Phase 11, step 93
• docs/plan/07-operations-testing.md#test-strategy
• docs/plan/02-resolver-cache.md#cache-design
• docs/plan/05-persistence-config.md#configuration-reload