decouple malachite consensus from engine API

ref: ADR-0001 (#226)

emerald currently has malachite consensus directly calling Engine API methods (`generate_block`, `notify_new_block`, `set_latest_forkchoice_state`, etc.) throughout `app/src/`. this couples consensus logic to EVM execution details like retry config, fee recipient, fork detection, payload caching, and validator contract reads.

introduce a consensus-agnostic `ExecutionLayer` trait in a new `crates/core` crate as the single integration boundary. restructure the codebase so malachite consensus is generic over this trait and the EVM Engine API is one concrete implementation behind it.

### crate layout after

```
crates/
core/ # ExecutionLayer trait (no malachite, no alloy deps)
consensus/malachite/ # malachite consensus, generic over ExecutionLayer
execution/evm/ # Engine API impl of ExecutionLayer
cli/ # single binary wiring everything together
```

existing `emerald start` behavior is preserved throughout. each PR compiles and passes tests independently.