driftbrake 0.2.0

Chain-agnostic pre-flight simulation + drift-based halt guard for simulation-driven trading strategies. Re-exports core, reconcile, and receipt-poller as a single dependency. The REVM simulation backend (driftbrake-revm-backend) is a separate, opt-in crate, not bundled here — see docs/ARCHITECTURE.md's 'swappable simulation backend' goal: a team using their own simulation stack should not be forced to compile REVM just to depend on driftbrake.