Add loom tests to sharded-slab #8

Context: sharded-slab is concurrent by design but has zero loom coverage. Template exists in lib/spin/src/mutex.rs (look for #[cfg(loom)] modules).

Scope:

• Add a #[cfg(loom)] test module exercising concurrent insert/remove across threads.
• Add a loom_test target to lib/sharded-slab/BUCK.
• Verify it catches a deliberately-broken ordering.

Acceptance: just test lib/sharded-slab includes the loom target; test passes.