- Add src/adapters.ts with nobleECPair (ECPairAPI) and scureBIP32 (BIP32API)
  built-in implementations using the packages this library already depends on
- Make DescriptorsFactory params optional, defaulting to built-in adapters
- Export defaultFactory for zero-config usage: DescriptorsFactory()
- Re-export ECPairAPI, ECPairInterface, BIP32API, BIP32Interface types
- Simplify test/helpers/crypto.ts to re-export from src/adapters.ts

Consumers no longer need to write ~170 lines of adapter boilerplate.

Closes #2