Create a fresh static change address for fractional loop-ins and persist
its key locator with the selected HTLC outpoint. Recovery reconstructs
the same change output instead of returning funds to the legacy root
address.
The Parameters struct describes the keys, expiry and pkScript that
define the static address script, so its natural home is the script
package. Moving it there lets staticutil drop its dependency on the
address package and lets callers reuse a single type alongside
script.StaticAddress and script.NewStaticAddress.
No behavior change.
Closes#1056
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.
