Handle wallet output descriptor when derivation path is not provided

[macaronish]

When the derivation path is not provided in the output descriptor for one of the signers (see example provided by @jlopp for Bluewallet), Sparrow should assume path 'm/0/...', matching the path length of the other signers ('m/0/0/0' for the example above). Path length matching seems to matter for Coldcard because it can't sign PSBTs created by multisig wallets with mismatched derivation path lengths.

I would say this is low priority since this seems to be an issue with only Casa at the moment and they could just easily add the full derivation path for all signers when exporting the output descriptor.

[craigraw]

Sparrow does "handle" the wallet output descriptor when derivation path is not provided - it simply assumes the xpub is derived directly from the xprv, which contains the least assumptions. Matching the length of other derivation paths in a multisig seems to contain more assumptions to me. If there is some kind of industry standard around this with a good reasoning I'd be happy to consider it, but I'm not aware of such.

As you suggest, it's probably better for Casa to add some kind of derivation path (even if it's not the actual derivation path) to their exports.

[craigraw]

Closing this off.