Harden cold archive consistency

[GatewayJ]

Summary

• make persistent single-node metadata Raft the default path and add an atomic CompleteArchiveObject CAS-style metadata update
• harden cache capacity handling so staging pressure is rejected explicitly and restored eviction cannot mask staging budget exhaustion
• expand gateway/scheduler protocol behavior for S3 errors, restore/list semantics, archive validation, and concurrent task boundaries
• refresh architecture/module documentation to match the current implementation

Validation

• pre-commit hook passed:
  • cargo fmt --all -- --check
  • cargo clippy --workspace --all-targets -- -D warnings
  • cargo check --workspace
  • cargo test --workspace --no-fail-fast