Include interactive funding candidates on broadcast

[jkczyz]

Include every negotiated funding candidate (original + RBF replacements) at broadcast time so downstream consumers can update their own state tracking from the broadcaster callback. The local contribution data isn't recoverable from the on-chain transaction, and the replaced txids aren't either, so the broadcaster callback is the only place where this information can be observed.

• TransactionType::Splice is replaced with TransactionType::InteractiveFunding { candidates: Vec<FundingCandidate> }. Each FundingCandidate carries its txid and the channels participating in it; each ChannelFunding carries the channel id, counterparty node id, purpose (Establishment / Splice), and the local node's contribution for this candidate. The list covers every negotiated candidate in order — original first, then each RBF replacement — letting downstream reconcile any historical txid, not just the immediate predecessor. The alternative (reacting to SplicePending) was worse: that event races with BDK wallet sync and doesn't carry the replaced txid.
• The new variant is structured to be forward-compatible with batches and V2 (dual-funded) channel establishment, neither of which is implemented today. For the current single-channel splice path, each candidate's channels list has length 1.
• FundingCandidate, ChannelFunding, and FundingPurpose implement Writeable / Readable so downstream can persist them directly without mirroring.
• FundingContribution gains public getters for estimated_fee, inputs, and max_feerate; feerate is elevated from pub(super) to pub. Combined with existing value_added, outputs, change_output, this exposes everything a downstream consumer needs without reaching into the raw transaction.