Add JCS + BLAKE3 receipt vectors for Prophet evidence lane

[mdheller]

Summary

Add receipt/content hashing vectors for the Prophet evidence lane using the repository's stated JSON canonicalization posture: RFC 8785 JCS canonical JSON plus BLAKE3-256.

Depends on #50.

Proposed changes

• Add canonical receipt examples for evidence submission, verification, and promotion receipts.
• Add JCS canonical byte examples.
• Add BLAKE3-256 digest vectors.
• Document the distinction between transport fixture vectors and receipt/content-hash vectors.

Acceptance criteria

• New vectors are under an agreed fixture path, likely fixtures/jcs_blake3_prophet_lane1.json or a successor path chosen by maintainers.
• Vectors are verified by an automated check.
• Documentation explains that sorted JSON + SHA-256 examples in earlier white-paper drafts were pedagogical only, not final receipt compliance.

Boundary rule

This issue is receipt/canonicalization support. It should not redefine Prophet evidence semantics.