SP Change Address

[notTanveer]

Implements BIP-352 label-0 for change addresses. Instead of sending change back to the regular wallet address, change is now sent to a labeled silent payment address (label = 0), making it indistinguishable from normal silent payment outputs on-chain.

Changes

• Added labeled spend key derivation (SP_CHANGE_LABEL = 0) with helpers for deriving both private and public keys.
• Updated the Rust scanner to scan against multiple spend pubkeys, return the matching key, and expose it as matchedSpendPubkey.
• Updated transaction processing to scan for both the primary and change spend pubkeys, and assign the correct silentPaymentAddress to matched UTXOs.
• Updated createSPTransaction to use the appropriate spend key (main or labeled change) when tweaking keys, generating SP outputs, and signing.
• Fixed broadcastTx so the post-broadcast scan correctly recognizes change outputs sent to the labeled change address and includes them in balance calculations.
• Updated SendDetails to automatically use the labeled silent payment change address whenever the recipient is an sp1 address.