KeychainTxOutIndex returns stale SPK cache entries in per-call ChangeSets

[ValuedMammal]

Describe the bug
KeychainTxOutIndex stages newly derived script pubkeys in an internal staging buffer and the flushes that buffer into the next returned ChangeSet. As a result, per-call ChangeSet results aren't operation-local: a call such as reveal_next_spk could return SPK cache entries derived by earlier operations (for example descriptor insertion / index store replenishment), which is surprising for downstream consumers that persist deltas per API call.

To Reproduce

1. Create KeychainTxOutIndex with persist_spks = true and a nonzero lookahead.
2. Insert a descriptor.
3. Call reveal_next_spk once.
4. Inspect the returned ChangeSet.spk_cache.

Expected behavior
Each mutating API call should return a ChangeSet containing only the cache delta created by that call. For example, reveal_next_spk should return only newly derived scripts from that reveal step, not previously staged scripts from unrelated prior calls.

Build environment

• BDK tag/commit: 47556ab
• OS+version:
• Rust/Cargo version: rustc 1.94.1
• Rust/Cargo target:

Which backend(s) are relevant (if any)?

• Electrum
• Esplora
• Bitcoin Core RPC
• None / not backend-related (e.g. bdk_chain, bdk_core)
• Other (please specify): ____

Is this blocking production use?

• Yes
• No