feat: implement Phase 1 Axon protocol core package

Adds the foundational Go package implementing the full Axon protocol
signing and crypto spec per PROTOCOL.md:

- Event/Tag structs and all kind constants (KindProfile through KindJobFeedback)
- Byte-exact canonical_payload construction per the PROTOCOL.md layout table
- Tag sorting and canonical_tags SHA256 hash (duplicate detection included)
- Ed25519 sign/verify, challenge sign/verify
- X25519 key conversion from Ed25519 keypair (RFC 8032 §5.1.5 clamping +
  birational Edwards→Montgomery map for pubkeys)
- ChaCha20-Poly1305 encrypt/decrypt for DMs (nonce prepended)
- MessagePack encode/decode for events and wire messages

Test vectors written first in testdata/vectors.json covering canonical_tags,
canonical_payload, event_id, and signature verification — all deterministic
known-input → known-output pairs for cross-language validation in Phase 4.

13 tests, all passing.

0 files changed, 0 insertions, 0 deletions