## Summary

- Adds an opt-in `/lean/v0/test_driver/...` HTTP router gated by
`HIVE_LEAN_TEST_DRIVER`, enabling the new lean spec-asset suites from
[ethereum/hive#1480](ethereum/hive#1480)
(`lean-spec-tests-{fork-choice,state-transition,verify-signatures}`) to
drive ethlambda over HTTP.
- In driver mode the binary skips the BlockChain actor and P2P swarm
entirely and serves only the four new endpoints plus `/lean/v0/health`.
State lives behind `Arc<RwLock<Store>>` so each `fork_choice/init`
request replaces the store and a single container can replay many
fixtures.
- Reuses the production code paths (`store::on_tick` /
`on_block_without_verification` / `on_gossip_*`,
`state_transition::state_transition`, `verify_block_signatures`) so the
driver exercises the same logic as the consensus stack.
- Lifts the fork-choice and verify-signatures fixture deserialization
types out of `crates/blockchain/tests/` into the shared
`ethlambda-test-fixtures` crate so the RPC handler and offline spec-test
runner consume the same JSON schemas.

### Endpoints (camelCase per the hive contract)

```
POST /lean/v0/test_driver/fork_choice/init      -> 204 / 400
POST /lean/v0/test_driver/fork_choice/step      -> { accepted, error?, snapshot }
POST /lean/v0/test_driver/state_transition/run  -> { succeeded, error?, post? }
POST /lean/v0/test_driver/verify_signatures/run -> { succeeded, error? }
```

### Hive run result

End-to-end run against a local hive instance with ethlambda built from
this branch — all three spec-asset suites green:

| Suite | Result |
|---|---|
| `lean-spec-tests-fork-choice`     | **85/85** ✅ |
| `lean-spec-tests-state-transition`| **51/51** ✅ |
| `lean-spec-tests-verify-signatures`| **12/12** ✅ |
| **Total**                          | **148/148** ✅ |

First fork-choice run was 84/85: a single fixture
(`test_store_from_anchor_rejects_mismatched_state_root`) caught a
missing anchor invariant in the init handler. Fix in `1aa9f8f` enforces
`block.state_root == state.tree_hash_root()` (with the header's
`state_root` zeroed), mirroring Ream's check, and a new e2e regression
test (`init_with_garbage_block_state_root_returns_400`) covers the exact
shape. The rerun went 85/85.

### Commits

- `41b86fd` feat(rpc): add Hive lean spec-assets test-driver endpoints
- `b1f2ba5` fix(bin): check `HIVE_LEAN_TEST_DRIVER` before reading
`--node-key`
- `1aa9f8f` fix(rpc): reject anchors whose `block.state_root` disagrees
with the state

## Test plan

- [x] `cargo fmt --all -- --check`
- [x] `cargo clippy --workspace --tests --all-targets -- -D warnings`
- [x] `cargo test -p ethlambda-rpc` — 18 lib + 8 e2e tests (the 8 e2e
tests use `tower::ServiceExt::oneshot` to exercise every endpoint with
real JSON bodies; one of them is a regression test for the anchor
invariant from `1aa9f8f`)
- [x] `cargo test --workspace --lib --bins` — all suites green
- [x] Smoke test the binary in driver mode with a non-existent
`--node-key`; confirm short-circuit + `/lean/v0/health` +
`/state_transition/run` respond correctly
- [x] Hive `lean-spec-tests-fork-choice` — **85/85 pass**
- [x] Hive `lean-spec-tests-state-transition` — **51/51 pass**
- [x] Hive `lean-spec-tests-verify-signatures` — **12/12 pass**