`LocalChain::apply_changeset` silently replaces the genesis block

**Describe the bug**

`LocalChain::apply_changeset` accepts a `ChangeSet` containing `(0, Some(hash))` with a hash different from the chain's current genesis and silently rewrites the genesis block. `apply_changeset_to_checkpoint` (`crates/chain/src/local_chain.rs`) collects the existing checkpoints from the changeset's lowest height upwards, overwrites height 0 with the changeset entry, and rebuilds the chain via `LocalChain::from_blocks`, which never compares against the previous genesis.

The other mutation entrypoints enforce this invariant: `apply_update` (via `merge_chains`) returns `CannotConnectError` when the update disagrees on genesis, and `insert_block` / `disconnect_from` also refuse to alter height 0. `apply_changeset` is the only one that does not, so a persisted or externally constructed changeset with a wrong height-0 entry can move a chain onto a different genesis (e.g. another network) without any error.

This issue was found by AI.

**To Reproduce**

Add `crates/chain/tests/test_genesis_changeset.rs` and run `cargo test -p bdk_chain --test test_genesis_changeset`:

```rust
use bdk_chain::local_chain::{ChangeSet, LocalChain};
use bdk_testenv::{hash, local_chain};
use bitcoin::BlockHash;

#[test]
fn apply_changeset_does_not_replace_genesis() {
let mut chain: LocalChain = local_chain![(0, hash!("G")), (1, hash!("A"))];
let changeset: ChangeSet = [(0, Some(hash!("not_G")))].into_iter().collect();

let result = chain.apply_changeset(&changeset);

assert_eq!(chain.genesis_hash(), hash!("G"), "apply_changeset returned {result:?}");
}
```

Current output:

```
assertion `left == right` failed: apply_changeset returned Ok(())
left: 7fa96a46d03a598e64ddfaadb1525023753ae80470cacac37ebb5fbc837ebeb9
right: dc4e3f5d6f1c6736abbdfa7a86e999ebdc7b132b82411571fc3909ac0ebc41fa
```

**Expected behavior**

`apply_changeset` should not silently replace an existing genesis block; a changeset that disagrees on height 0 should be handled consistently with the other `LocalChain` mutation methods.

Start in crates/chain/src/local_chain.rs, focusing on apply_changeset_to_checkpoint and LocalChain::apply_changeset, then add the reproduction in crates/chain/tests/test_genesis_changeset.rs. Run cargo test -p bdk_chain --test test_genesis_changeset; done means a changeset with a conflicting height-0 hash is rejected consistently with the other mutation methods and the original genesis remains unchanged.

Written by the indexing model from the issue text.