Fix orphaned mempool_rbf_tests: wire module and live APIs

[secsovereign]

Summary

• tests/unit/mempool_rbf_tests.rs was orphaned (not in tests/unit/mod.rs) and called the old 3-arg replacement_checks.
• Wire mod mempool_rbf_tests; and update call sites to replacement_checks(&rbf_tx, &non_rbf_tx, &utxo, &pool) (pass UtxoSet).
• Import Network; align duplicate detection with MempoolResult::Rejected (seed mempool — accept_to_memory_pool does not mutate it).
• Does not pull other orphaned unit modules into this PR. No PARTIAL_BASELINE bump, no merge/publish.

Test plan

• cargo fmt / cargo fmt --check
• CARGO_INCREMENTAL=0 cargo check (path patch aside → crates.io; restored after)
• cargo test --test unit_suite unit::mempool_rbf_tests:: → 2 passed (rbf sequence / duplicate detection)