Projection Determinism Golden Vectors (JCS)

[flyingrobots]

Goal\nEnsure canonical bytes are stable across platforms/encoders when computing content_id and pointer digests.\n\nWhy\nDeterminism is normative (RFC 8785 JCS + key ordering). We need confidence via golden vectors.\n\nAcceptance Criteria\n- Add cross-platform JCS golden vectors for pointers and governance PoC envelopes.\n- CI job runs hash checks on Linux/macOS/Windows matrix.\n- Mixed JSON encoders (serde_json, simdjson wrappers, etc.) produce identical canonical bytes after JCS.\n- Document how to regenerate vectors.\n\nNotes\n- Reference: ADR-0004 determinism section; SPEC Canonicalization rules.\n- Include example with large maps to exercise ordering.\n