Fix async runtime usage: tokio::sync::Mutex for WAL + tooling config

[meonthewire]

Summary

WAL mutex blocks the async runtime

The write-ahead log is wrapped in Arc<std::sync::Mutex<Wal>>, which means every lock acquisition blocks the Tokio runtime thread while disk I/O completes. Under concurrent load, this can stall all tasks on that worker thread, degrading throughput and latency for unrelated operations.

No project-wide clippy or formatting configuration

The project has no clippy.toml or rustfmt.toml, meaning there are no shared lint thresholds or formatting rules.

Acceptance Criteria

• No std::sync::Mutex used in async code paths
• cargo fmt --all --check passes
• cargo clippy --all-targets -- -D warnings passes