DIP: Self-Describing Hash Format for Intel DCAP reportdata

We propose a new standard format for encoding hashes in Intel DCAP attestation `reportdata`, which is limited to 64 bytes. The format is:

```
::
```

- Purpose: short human-readable identifier (≤10 chars).
- Algo: algorithm name (e.g., sha-256).
- Digest: Base64url-encoded hash, without padding.

This format is inspired by existing practices (ni://, SRI, Docker digests), but tailored for the strict 64-byte size constraint. It is primarily intended for SHA-256, since longer digests would overflow.

Advantages:
- Self-describing (algorithm embedded).
- Compact (Base64url shorter than hex).
- Domain separation via purpose prefix.
- Reusable tooling: can leverage existing ni:// and SRI libraries for serialization/parsing.

PR: #331