createHtlcTx always places the HTLC output at index 0 and the change
output (if any) at index 1. Previously, createHtlcSweepTx attempted to
dynamically find the HTLC index but then unconditionally read
TxOut[0].Value, ignoring the computed index.
Replace the dynamic search with a const htlcInputIndex=0 and a fail-fast
check that errors if the layout invariant is ever violated. Add a test
that verifies the sweep value is derived from the HTLC output, not the
change output.
