Add comprehensive spec audit results and pyMC_core issue reports

- CHANGELOG.md: Full audit findings organized by severity
- PYMC_CORE_ISSUES.md: 7 evidence-based bug reports for pyMC_core
  (wrong constants, broken path_len decoding, wrong TRACE hash,
  wrong SNR calculation)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Add comprehensive spec audit results and pyMC_core issue reports

- CHANGELOG.md: Full audit findings organized by severity
- PYMC_CORE_ISSUES.md: 7 evidence-based bug reports for pyMC_core
  (wrong constants, broken path_len decoding, wrong TRACE hash,
  wrong SNR calculation)
- Remove ERRATA.md: contents already covered by audit findings

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

spec/07-payload-anon-req.md

@@ -0,0 +1,74 @@
+# MeshCore Protocol Specification
+
+## Section 7: Payload — Anonymous Request
+
+### Overview
+
+The anonymous request payload allows a node to send an encrypted message to a
+recipient without the recipient needing to know the sender in advance. Instead of
+a 1-byte source hash, the full 32-byte Ed25519 public key of the sender is
+included, enabling the recipient to compute the shared secret for decryption.
+
+### Payload Type
+
+Header payload type field: `0x07` (ANON_REQ)
+
+### Wire Format
+
+```
+ 0
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+| dest_hash(1)  |                                               |
++-+-+-+-+-+-+-+-+                                               |
+|                   Sender Public Key (32 bytes)                |
+|                                                               |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+|    Cipher MAC (2 bytes)       |                               |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
+|              Ciphertext (N × 16 bytes, N ≥ 1)                |
+|                                                               |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+```
+
+### Fields
+
+| Field | Offset | Size | Type | Description |
+|-------|--------|------|------|-------------|
+| dest_hash | 0 | 1 byte | raw | First byte of recipient's Ed25519 public key |
+| sender_pub_key | 1 | 32 bytes | raw | Full Ed25519 public key of sender |
+| cipher_mac | 33 | 2 bytes | raw | Truncated HMAC-SHA256 over ciphertext |
+| ciphertext | 35 | variable (multiple of 16) | raw | AES-128-ECB encrypted data |
+
+### Minimum Size
+
+- Minimum payload: 1 + 32 + 2 + 16 = 51 bytes
+
+### Encryption
+
+1. Sender computes ECDH shared secret using its private key and the recipient's
+   public key.
+2. Encrypt plaintext using AES-128-ECB with first 16 bytes of shared secret.
+3. Compute HMAC-SHA256 over ciphertext with full 32-byte shared secret.
+   Truncate to 2 bytes.
+4. Assemble: dest_hash || sender_pub_key || MAC || ciphertext.
+
+### Decryption
+
+1. Read dest_hash. If it does not match this node, the packet is not for us
+   (but MAY be forwarded).
+2. Read 32-byte sender_pub_key. Construct an Identity from it.
+3. Compute ECDH shared secret between this node's private key and sender's
+   public key.
+4. Verify the MAC and decrypt as in [Section 14](14-crypto.md).
+
+### Cross-References
+
+- [Section 6: Encrypted Payloads](06-payload-encrypted.md) — Standard encrypted format
+- [Section 14: Cryptography](14-crypto.md) — Encrypt-then-MAC
+- [Section 15: Identity](15-identity.md) — ECDH key exchange
+- Test vectors: `corpus/payloads/anon-req/`
+
+### Reference Implementation
+
+- `Mesh::createAnonDatagram()` in `src/Mesh.cpp` — Encoding
+- `Mesh::onRecvPacket()`, case `PAYLOAD_TYPE_ANON_REQ` — Decoding