Subsidize RBF

[olga24912]

Summary

Adds a Subsidize RBF flow so the protocol (or any account) can push
through a stuck BTC withdrawal by topping up its gas fee with extra nBTC.
The user still receives the same BTC amount — only the miner fee grows.

Motivated by UX: when an operator sees that a user's withdrawal is not
confirming, they can subsidize an RBF replacement instead of waiting.

How it works

The subsidizer calls ft_transfer_call with the new message
TokenReceiverMessage::Rbf { pending_tx_id, output }. The bridge
constructs an RBF replacement of the stuck tx, routing the subsidy
entirely into gas fee. Invariants enforced:

• actual_received_amount unchanged.
• gas_fee grows by exactly the subsidy amount.

Emits a new SubsidizeRbf event.

Zcash version is unimplemented! for now.

Migration notes

OriginalState gains a new field subsidize_amount: u128. Existing
pending infos are migrated on upgrade via
migrate_btc_pending_infos_to_current in legacy.rs, defaulting
subsidize_amount = 0 for pre-existing entries. No off-chain action
required; integrators only need to be aware of the new Rbf message
variant and the SubsidizeRbf event.