Loop-in and loop-out responses carry compressed server public keys that are copied into fixed-size fields and later used for HTLC construction. Validate the length and parse each compressed key before storing it, and validate the MuSig2 loop-in receiver internal key as well.
This turns short or unparsable server keys into explicit errors instead of silently zero-padding short responses or accepting an invalid internal key. Update root test mocks to return size-correct MuSig2 signing data under the stricter checks.
