Support EIP-7843 slot_num in BlockEnv for OP Stack

## Summary

The `revm` upgrade to v36 introduced a `slot_num` field on `BlockEnv` (from [EIP-7843](https://eips.ethereum.org/EIPS/eip-7843)). This field is currently hardcoded to `0` in `alloy-op-evm` since EIP-7843 is not yet enabled in the OP Stack.

## Context

In `alloy-op-evm/src/env.rs`, the `BlockEnv` constructor sets `slot_num: 0`. Once EIP-7843 is activated in the OP Stack, this field should be populated from the block header.

## Action Items

- [ ] Track when EIP-7843 is scheduled for the OP Stack
- [ ] Add `slot_num` to `EvmEnvInput` and populate it from `BlockHeader` once available
- [ ] Update `evm_env_for_op_payload` to extract `slot_num` from the execution payload

Start in alloy-op-evm/src/env.rs by reading the BlockEnv constructor and tracing EvmEnvInput through evm_env_for_op_payload. Check how BlockHeader and the execution payload expose slot_num once EIP-7843 is scheduled for the OP Stack. Done means slot_num is carried through EvmEnvInput and populated from the payload instead of remaining hardcoded to 0.

Written by the indexing model from the issue text.