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/08-payload-group.md

@@ -0,0 +1,81 @@
+# MeshCore Protocol Specification
+
+## Section 8: Payload — Group (GRP_TXT / GRP_DATA)
+
+### Overview
+
+Group payloads enable encrypted communication within a channel. All members of a
+channel share a symmetric secret. The payload begins with a 1-byte channel hash
+(derived from the channel's shared key), followed by an encrypt-then-MAC blob.
+
+### Payload Types
+
+| Value | Name | Description |
+|-------|------|-------------|
+| 0x05 | GRP_TXT | Group text message |
+| 0x06 | GRP_DATA | Group datagram (binary data) |
+
+### Wire Format
+
+```
+ 0
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+|channel_hash(1)|    Cipher MAC (2 bytes)       |               |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
+|              Ciphertext (N × 16 bytes, N ≥ 1)                |
+|                                                               |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+```
+
+### Fields
+
+| Field | Offset | Size | Type | Description |
+|-------|--------|------|------|-------------|
+| channel_hash | 0 | 1 byte | raw | First byte of SHA-256(channel_shared_key) |
+| cipher_mac | 1 | 2 bytes | raw | Truncated HMAC-SHA256 over ciphertext |
+| ciphertext | 3 | variable (multiple of 16) | raw | AES-128-ECB encrypted data |
+
+### Minimum Size
+
+- Minimum payload: 1 + 2 + 16 = 19 bytes
+
+### Channel Hash Derivation
+
+The channel_hash is the first byte of the SHA-256 hash of the channel's 32-byte
+shared key (PUB_KEY_SIZE bytes):
+
+```
+channel_hash = SHA256(channel_shared_key)[0]
+```
+
+This means hash collisions are expected (1 in 256 chance). When a receiver finds
+a matching channel_hash, it attempts decryption with each matching channel's
+secret. Only a successful MAC verification confirms the correct channel.
+
+### Encryption
+
+1. Encrypt plaintext using AES-128-ECB with first 16 bytes of the channel's
+   shared key (GroupChannel.secret).
+2. Compute HMAC-SHA256 over ciphertext using all 32 bytes of the channel's
+   shared key. Truncate to 2 bytes.
+3. Assemble: channel_hash || MAC || ciphertext.
+
+### Decryption
+
+1. Read channel_hash (1 byte).
+2. Search local channel database for channels with matching hash. Multiple
+   channels may match (up to 4 in reference implementation).
+3. For each matching channel, attempt MAC verification and decryption using the
+   channel's shared key.
+4. The first channel that produces a valid MAC yields the correct decryption.
+
+### Cross-References
+
+- [Section 14: Cryptography](14-crypto.md) — Encrypt-then-MAC
+- Test vectors: `corpus/payloads/group/`
+
+### Reference Implementation
+
+- `Mesh::createGroupDatagram()` in `src/Mesh.cpp` — Encoding
+- `Mesh::onRecvPacket()`, cases `PAYLOAD_TYPE_GRP_TXT/GRP_DATA` — Decoding
+- `Mesh::searchChannelsByHash()` — Channel lookup