A verification method can only hold one purpose, and createVMID is not exported

`VerificationMethod.purpose` is a single `DataIntegrityProofPurpose`, and `normalizeVMs` pushes each method into exactly one relationship. There is no way to express one key that is both `authentication` and `assertionMethod` — the common shape for a DID with a single key.

The workaround is passing explicit `authentication: [...]` / `assertionMethod: [...]` arrays, but those need the verification method's id. `createVMID` derives it as `${did}#${publicKeyMultibase.slice(-8)}`, and at create time the caller doesn't know the DID yet (the SCID isn't derived). `createVMID` also isn't re-exported from `src/index.ts`, so callers either re-implement the fragment derivation or pass a relative `#` reference and hope the derivation doesn't change.

credo-ts does the latter: openwallet-foundation/credo-ts#2928

Suggestion: accept `purpose` as an array (or add `purposes`), so one method can land in several relationships without the caller needing to construct its id.