In order to reuse CoinSelectionSource for splicing, the previous
transaction of each UTXO is needed. Update CoinSelection to use
FundingTxInput (renamed to ConfirmedUtxo) so that it is available.

This requires adding a method to WalletSource to look up a previous
transaction for a UTXO. Otherwise, Wallet's implementation of
CoinSelectionSource would need WalletSource to include the previous
transactions when listing confirmed UTXOs to select from. But this would
be inefficient since only some UTXOs are selected.

CoinSelectionSource is used for anchor bumping where a ClaimId is passed
in to avoid double spending other claims. To re-use this trait for
funding a splice, the ClaimId must be optional. And, if None, then any
locked UTXOs may be considered ineligible by an implementation.

Previously, splice_channel required callers to manually construct
funding inputs and pass them directly, making coin selection the
caller's responsibility. This made the API difficult to use and
prevented reuse of the existing CoinSelectionSource trait.

Introduce a two-phase API: splice_channel now returns a FundingTemplate
that callers use to build a FundingContribution via wallet-backed
splice methods (e.g., splice_in_sync, splice_out_sync), which handle
coin selection automatically. The completed contribution is then passed
to a new funding_contributed method to begin quiescence and negotiation.

This also renames SpliceContribution to FundingContribution and moves
fee estimation and input validation into the funding module, co-located
with the types they operate on.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Now that CoinSelection is used to fund a splice funding transaction, use
that for determining of a change output should be used. Previously, the
initiator could either provide a change script upfront or let LDK
generate one using SignerProvider::get_destination_script.

Since older versions may have serialized a SpliceInstruction without a
change script while waiting on quiescence, LDK must still generate a
change output in this case.

Instead of logging both inside propose_quiescence and at the call site,
only log inside it. This simplifies the return type.

Wallet-related types were tightly coupled to bump_transaction, making
them less accessible for other use cases like channel funding and
splicing. Extract these utilities to a dedicated module for improved
code organization and reusability across the codebase.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Synchronous wallet utilities were coupled to bump_transaction::sync,
limiting their reusability for other features like channel funding and
splicing which need synchronous wallet operations. Consolidate all
wallet utilities in a single module for consistency and improved code
organization.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

FundingTxInput was originally designed for channel funding but is now
used more broadly for coin selection and splicing. The name
ConfirmedUtxo better reflects its general-purpose nature as a confirmed
UTXO with previous transaction data. Make ConfirmedUtxo the real struct
in wallet_utils and alias FundingTxInput to it for backward
compatibility.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>