Build splice_in_with_all from manually selected inputs

[jkczyz]

Fixes #1168.

Node::splice_in_with_all estimated the maximum amount with a wallet drain and then ran coin selection for that exact amount. The resulting contribution had at most the anchor reserve as a change output, so bump_channel_funding_fee had nothing else to pay a higher fee from. With anchor channels enabled, the bump shrank the change output and spent the reserve on the splice fee. Without a reserve there was no change output at all, so the bump failed with ChannelSplicingFailed. The drain estimate also ran at the funding feerate estimate while the contribution could be built at a pending splice's higher RBF minimum.

This PR instead hands LDK the wallet's confirmed UTXOs as manually selected inputs (FundingBuilder::add_inputs). They are consumed in full with no change output, the amount added to the channel is whatever remains after fees at the template's feerate, and a later fee bump is paid by adding a little less to the channel. When an anchor reserve is required, it is kept as an explicit output back to one of the wallet's own addresses, which stays fixed across bumps.

The reserve address is marked used in BDK as soon as it is handed out, since the splice transaction only reaches the wallet once negotiation completes and until then BDK would offer the same address as change elsewhere. The mark is dropped again if the contribution cannot be built, or via DiscardFunding if LDK rejects it.

Insufficient-funds checks stay in front of LDK, which would otherwise build a near-empty contribution. The drain-based splice estimate is removed along with the shared-input handling in the drain helpers that only it used.

The first commit extracts the low -incrementalrelayfee bitcoind setup used by the splice RBF tests into a shared helper, since the new tests need it too.

Tests

• splice_in_with_all_bump_fee_retains_anchor_reserve: splice in with all funds with an anchor reserve, bump the fee, and check the bump succeeds, the amount added to the channel drops by exactly the fee increase, and the on-chain balance after confirmation still holds the reserve.
• splice_in_with_all_bump_fee_without_anchor_reserve: same without a reserve, where the bump previously failed outright.
• A wallet unit test for the confirmed-input listing (unconfirmed outputs excluded) and one for the address reservation (skipped while reserved, offered again once released).

Both integration tests fail on main: with a reserve the bump leaves the spliced-in amount unchanged while the fee rises, i.e. it is paid from the reserve; without a reserve bump_channel_funding_fee returns ChannelSplicingFailed.

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