Expanding the findings from #71 .

Add SDK-level #[non_reentrant] modifier for ALLOW_REENTRY opt-in contracts

Context

When a contract opts into ALLOW_REENTRY, pallet-revive's default reentrancy reject is bypassed (see #67 ). The contract author now needs to write their own guard, typically a bool locked slot with check-set-clear around the reentrant entry point. This is exactly the OZ nonReentrant pattern.

Notice that "Reentrancy-protected by default" and #[non_reentrant] are two different mechanisms. By default pallet-revive rejects reentrant calls at the runtime/host level, which costs zero contract bytecode . So the typical contract is already protected and pays nothing. #[non_reentrant] exists only for the minority of contracts that deliberately turn that default off via CallFlags::ALLOW_REENTRY.

Scope

Add a method attribute:

#[method]
#[non_reentrant]
fn flash_loan(&mut self, ...) -> Result<(), Error> {
  // user body
}

This may use a hidden storage item _reentrancy_locked: Lazy<bool> slot at the contract layout, which is used as a lock shared across all #[non_reentrant] methods in the same contract