Release RustQueue 0.8.4

[SamuelSupe]

Summary

• Batch durable Channel FIN/REQ writes into bounded 1 ms / 1,024-request WAL group commits.
• Release the Topic state lock during Channel WAL fsync while preserving serialization with a dedicated Channel commit gate.
• Remove hot-path allocations with stack WAL encoding, shared Topic/Channel identities, unboxed Channel operation futures, and a 256-operation active window per TCP session.
• Harden benchmark completion by waiting for server EOF and verifying final Channel drain.
• Prepare all RustQueue 0.8.4 release metadata, Helm/Console versions, README, Pages install command, and release notes.

Durability and compatibility

The v7 storage format is unchanged. Successful FIN and REQ responses still follow fsync of every affected durable Channel WAL. At-least-once delivery and the share-nothing single-PVC model are unchanged.

Validation

• ./scripts/verify-release-version.sh 0.8.4
• OrbStack release gate stages passed: fmt, all-feature check, workspace tests, Clippy, Console UI typecheck/tests/build, Helm lint/template
• Queue: 105 unit, 4 crash-boundary, 1 large-message, 8 recovery tests
• Server: 79 tests
• Go/Python core and TLS compatibility matrices passed
• Local fuzz image pull was blocked twice by a Docker Hub token endpoint timeout; this PR GitHub Actions production-gate is the authoritative full rerun, including fuzz smoke

Kubernetes acceptance is not repeated because this release does not change Kubernetes behavior.