fix(rbf): include evicted descendants in replacement fee floor

[noahjoeris]

Description

Problem: When replacing a tx that has unconfirmed descendants in the mempool, those descendants are evicted along with it but their fees were not counted toward the replacement's required fee. This allowed under-paying replacements that a node would reject.

Fix:

• Evicted descendants now count toward the replacement fee floor; previously their fees were dropped.
• RbfSet::new gains a descendants parameter -> breaking change
• Descendant traversal: Vec::contains (O(n)) → HashSet (O(1)).