Emitter silently replaces the caller’s genesis on a network mismatch

**Describe the bug**

When `Emitter` cannot find any caller-provided checkpoint on the RPC node, it silently replaces its checkpoint chain with the node's genesis and continues emitting blocks. This happens with an ordinary wrong-network configuration, such as mainnet checkpoints connected to a regtest node.

The emitted checkpoints no longer connect to the supplied chain, contrary to `BlockEvent::checkpoint`'s documented guarantee. `LocalChain::apply_update` rejects them, but the emitter has already switched its internal chain without reporting the mismatch.

This issue was found by AI.

**To Reproduce**

1. Run a regtest Bitcoin Core node with at least one mined block.
2. Create a `LocalChain` from mainnet genesis and pass its tip to `Emitter::new` with that node's RPC client, `start_height = 1`, and `NO_EXPECTED_MEMPOOL_TXS`.
3. Call `next_block()`. It returns a regtest block instead of reporting the network mismatch. Its checkpoint chain ends at regtest genesis, and `connected_to()` points to that genesis.
4. Apply the emitted checkpoint to the original `LocalChain`; it returns `CannotConnectError` at height 0.

Confirmed with an RPC stub returning `-5` for the caller's genesis and serving a chain rooted at a different genesis.

**Expected behavior**

An endpoint on a different network should not silently replace the caller's chain root or produce events that violate the documented connection guarantee.