Function contractPreimage

The preimage for a provisioned descriptor: opts.stored when the artifact kept one, otherwise re-derived from the seed.

Resolves in one precedence order, mirroring the arms provisionClaimSecret chose between:

1. opts.stored — a caller-supplied P, or one from a wallet that could not derive. Whatever the wallet can do now, a stored P is the artifact's.
2. a per-artifact descriptor — derive at the pinned index.
3. opts.salt — derive from the salted message.
4. otherwise throw: a repeating descriptor with neither a stored preimage nor a salt has nothing to derive from that would not collide.