Add Loom tests and verification + ci

[willothy]

Use tokio-rs/loom for safety checking.

[ellipsis-dev]

🚀	This implementation plan was created by @ellipsis-dev

Tag me (@ellipsis-dev) in a comment with guidance if you want me to replan or to implement this plan.

Implementation Plan

Step 1: Add Loom as dev dependency

Path: Cargo.toml
Add loom = "0.5" under [dev-dependencies] in Cargo.toml.

Step 2: Add Loom tests for concurrency

Path: tests/loom_tests.rs
In tests/loom_tests.rs, add Loom tests for ShardMap and ShardSet using loom::model to simulate concurrent operations.

Step 3: Add Loom tests to CI

Path: .github/workflows/rust.yml
In .github/workflows/rust.yml, add a job for Loom tests, mirroring the tests job. Include steps to install dependencies and run cargo test with Loom configurations.

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