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.

1 files changed, 34 insertions, 0 deletions

diff --git a/testdata/vectors.json b/testdata/vectors.json
new file mode 100644
index 0000000..7325fa5
--- /dev/null
+++ b/testdata/vectors.json
@@ -0,0 +1,34 @@
+{
+  "tags_input": [
+    {
+      "name": "p",
+      "values": [
+        "alice"
+      ]
+    },
+    {
+      "name": "e",
+      "values": [
+        "root-id",
+        "root"
+      ]
+    },
+    {
+      "name": "e",
+      "values": [
+        "reply-id",
+        "reply"
+      ]
+    }
+  ],
+  "canonical_tags_hex": "00030001650002000000087265706c792d6964000000057265706c79000165000200000007726f6f742d696400000004726f6f74000170000100000005616c696365",
+  "canonical_tags_hash": "2813b33f953e03e88160c651cbd4feb000c37a9b25690cad22fa26eb272bc9e6",
+  "seed_hex": "0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f20",
+  "pubkey_hex": "79b5562e8fe654f94078b112e8a98ba7901f853ae695bed7e0e3910bad049664",
+  "created_at": 1700000000,
+  "kind": 1000,
+  "content_hex": "68656c6c6f2061786f6e",
+  "canonical_payload": "002079b5562e8fe654f94078b112e8a98ba7901f853ae695bed7e0e3910bad049664000000006553f10003e80000000a68656c6c6f2061786f6e2813b33f953e03e88160c651cbd4feb000c37a9b25690cad22fa26eb272bc9e6",
+  "event_id": "16744a01674a332bab4e8814500b56b4a3c907c154dca01099ba6ed3aaba24df",
+  "sig_hex": "0706e27133980aedb544b3618b4afc63d234fad37e80e67018132dac7aad6c12a0923b1494390103ef301b2066a7e62870a623c91f0d21c3d1af1f9ac322ff08"
+}
\ No newline at end of file