9ed80ebf52
- Replace literal 8/9 with crate::v8::VERSION / crate::v9::VERSION so the compat branch tracks future protocol bumps automatically. - Add tracing::debug! when a v9 client falls back to a v8 reply so the fleet rollout is observable. - Document removal trigger (TODO IPR-v9-rollout) tied to crate::v9::MIN_RELEASE_VERSION. - Unit tests cover: matching versions, v9-accepts-v8 compat, v8-rejects-v9, unrelated mismatch, empty payload.