fix(mempool): reconcile conflicts before insertion

[EddieHouston]

Summary

Prevent submitted replacement transactions from temporarily coexisting with the transactions they replace in electrs' local mempool.

• detect locally indexed transactions that spend the same prevouts as an incoming transaction or package
• follow mempool spend edges to collect and remove all descendants of those conflicts
• reconcile conflicts before mutating any indexes, while keeping the ownership-aware edge removal from fix(mempool): tolerate missing/foreign edges on eviction #234 as defense in depth
• reject an internally conflicting incoming batch before insertion
• cover both broadcast_raw and submit_package immediate-insertion paths through the shared Mempool::add() implementation

This prevents the inconsistent state tracked in #236, where txstore and history could contain two conflicting spenders while edges could represent only one.

Fixes #236

Tests

• cargo test --test rest test_rest_mempool_rbf_reconciled -- --nocapture
• cargo test --test rest -- --nocapture (26 passed)
• cargo check --tests
• cargo check --tests --features liquid

The REST regressions verify immediately after both raw and package broadcast—before the next periodic mempool sync—that the replaced transaction is absent and the replacement is queryable.