`TxGraph::insert_txout` overwrites existing floating txout but returns empty `ChangeSet`

**Describe the bug**
`TxGraph::insert_txout` on an outpoint already present as a floating txout overwrites it in memory but returns an empty `ChangeSet`.

- Debug: panics.
- Release: live graph holds the new `TxOut`, changeset is empty, so persisted state keeps the old one. `IndexedTxGraph::insert_txout` forwards the empty changeset, so the indexer never sees the replacement.

Contradicts the doc comment ("`ChangeSet` result will be empty if the `outpoint` ... already existed") and the assert's own message ("txout of the same outpoint should never change").

**To Reproduce**
Run with `--release` (debug panics on the assert instead):

```rust
use bdk_chain::{tx_graph::TxGraph, Merge};
use bitcoin::{hashes::Hash, Amount, OutPoint, ScriptBuf, TxOut, Txid};

let op = OutPoint::new(Txid::all_zeros(), 0);
let a = TxOut { value: Amount::from_sat(1_000), script_pubkey: ScriptBuf::new() };
let b = TxOut { value: Amount::from_sat(2_000), script_pubkey: ScriptBuf::new() };

let mut graph: TxGraph = TxGraph::default();
let _ = graph.insert_txout(op, a.clone());
let cs = graph.insert_txout(op, b);

assert!(cs.is_empty()); // passes: reports "no change"
assert_eq!(graph.get_txout(op), Some(&a)); // FAILS: graph now holds `b`
```

**Expected behavior**
First insert wins: an existing outpoint is never overwritten, and an empty changeset truthfully means no change.

**Build environment**
- BDK tag/commit: acc06e53

**Which backend(s) are relevant (if any)?**
- [x] None / not backend-related (e.g. `bdk_chain`, `bdk_core`)

**Is this blocking production use?**
- [x] No