Make sure explicit digest labeling when not aligning with `alg` in `pay`

All digest values in `pay`must align with `alg` unless explicitly denoted. (Digest serialization)

I don't think this library is covering that or needs to, but I should double check. We may want to write a canary test.

For example, these example values in `pay` need to be correctly interpreted by external systems, but not necessarily Coz, which poses a difficult question: if Coz isn't handling it, what's the best practice for third parties?
```
ES256:U5XUZots-WmQYcQWmsO751Xk0yeVi9XUKWQ2mGz6Aqg
SHA-256:oDBDAg4xplHQby6iQ2lZMS1Jz4Op0bNoD5LK3KxEUZo
```

Perhaps including a `ParseDigest` function that handles `:` is warranted.

Start by checking the library's existing digest serialization and parsing behavior against the `pay` examples for ES256 and SHA-256. Investigate whether a `ParseDigest` entry point is needed for `:` values and whether a canary test can verify external-system interpretation. Done means the expected handling is confirmed or the proposed behavior is clearly tested.

Written by the indexing model from the issue text.