Clarify certificate encoding format: base64(DER) vs base64(PEM)

[matlec]

Bug description

The Device Client Onboarding section states:

certificate - Base64-encoded X.509 certificate of the client

This is ambiguous. "Base64-encoded X.509 certificate" could mean:

1. base64(DER) - Raw DER bytes, base64-encoded (e.g., MIIDdz...)
2. base64(PEM) - PEM file base64-encoded again (e.g.,LS0tLS1CRUdJTi...)

The example in the spec shows "certificate": "MIIDdzCCAl+gAwIBAgIEb1..." which is base64(DER). The sandbox implementation uses base64(PEM).

Proposed fix

Recommendation: Standardize on base64(DER)

base64(DER) is smaller, JSON-friendly, and matches what other specs such as JWS use to encode certificates. base64(PEM) would double-encode the certificate with base64.

Suggested spec wording:

certificate - Base64-encoded DER X.509 certificate (raw binary, no PEM headers).

Anything else (optional)

No response