Bip370 PSBTv2 support

[jvgelder]

To get #2269 working, we’ll need BIP370 (PSBTv2) first, since Silent Payments fields depend on it.

I’ve started an early implementation here:
https://github.com/jvgelder/bip370

It’s not finished and not properly documented yet, but I’d really appreciate early feedback on direction before going further.

Specifically:

• API design – What interface would you prefer? (e.g. batch updates, parity with current bitcoinjs PSBT API)
• Validation behavior – How strict should it be? Preserve unknown fields?
• Scope – Should it verify derivation paths / script correctness, or stay purely structural?

Notes:

• Mostly aligned with BIP174 patterns, but with stricter validation and explicit errors.
• No PSBTv0 support (could be added if needed).

Would love thoughts on whether this direction makes sense.