Add clippy async-safety lints to catch Mutex held across .await

Add workspace-level clippy lints:
- `await_holding_lock`: warns when a std::sync::Mutex (or RwLock) guard
  is held across an .await point, which can cause deadlocks or block the
  tokio runtime.
- `await_holding_refcell_ref`: warns when a RefCell borrow is held across
  an .await point, which will panic if the future is polled from a
  different context.

These lints pass cleanly on the existing codebase - all current
std::sync::Mutex usage correctly drops guards before .await points.

https://claude.ai/code/session_0133EbxcuGyrYJNCtfpKRjyR

Author: claude

Cargo.toml

@@ -426,3 +426,5 @@ unexpected_cfgs = { level = "warn", check-cfg = [
 
 [workspace.lints.clippy]
 as_conversions = "warn"
+await_holding_lock = "warn"
+await_holding_refcell_ref = "warn"