`KeychainTxOutIndex` panics on overlapping descriptors (non-index-0 spk)

**Describe the bug**
`insert_descriptor` only checks `DescriptorId` uniqueness (spk at index 0). A collision at any other index is accepted but the spk is never registered for the new keychain.

Debug: panics in insert_descriptor. Release: Ok(true), then reveal_next_spk panics.

Doesn't affect the stock two-keychain path (`wpkh(xpub/0/*)` + `wpkh(xpub/1/*)`). Hits any `KeychainTxOutIndex` with overlapping descriptors, e.g. watching a single address alongside its account (`wpkh(xpub/0/3)` + `wpkh(xpub/0/*)`), in either order. With the wildcard inserted second, early reveals work and the panic occurs at the colliding index.

**To Reproduce**
Run with `--release`:

```rust
use bdk_chain::keychain_txout::KeychainTxOutIndex;
use bdk_chain::miniscript::{Descriptor, DescriptorPublicKey};
use std::str::FromStr;

const XPUB: &str = "tpubD6NzVbkrYhZ4XgiXtGrdW5XDAPFCL9h7we1vwNCpn8tGbBcgfVYjXyhWo4E1xkh56hjod1RhGjxbaTLV3X4FyWuejifB9jusQ46QzG87VKp";

// K1's only spk == K0's spk at index 3 (within lookahead).
let k0 = Descriptor::::from_str(&format!("wpkh({XPUB}/0/*)")).unwrap();
let k1 = Descriptor::::from_str(&format!("wpkh({XPUB}/0/3)")).unwrap();

let mut idx = KeychainTxOutIndex::::new(25, true);
assert_eq!(idx.insert_descriptor(0, k0), Ok(true));
assert_eq!(idx.insert_descriptor(1, k1), Ok(true)); // accepted, but no spk registered for keychain 1
let _ = idx.reveal_next_spk(1); // panics: keychain_txout.rs:906 "we just inserted it"
```

**Expected behavior**
Behave as documented: first keychain to register an spk owns it, the other keychain keeps working, no panic in debug or release. Add tests for both insertion orders.

**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

Start in keychain_txout.rs at insert_descriptor and reveal_next_spk around the reported line 906, then trace how overlapping script pubkeys are registered. Add coverage for both descriptor insertion orders and verify that the first keychain owns the overlapping script, the other continues working, and neither debug nor release execution panics.

Written by the indexing model from the issue text.