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>

CLAUDE.md

@@ -0,0 +1,42 @@
+# MeshCore Spec — Project Guide
+
+## What This Project Is
+
+A specification and conformance test suite for the MeshCore mesh networking
+protocol. It contains:
+
+1. RFC-style spec documents in `spec/` (Markdown, using RFC 2119 language)
+2. JSON test vector corpus in `corpus/`
+3. Validation tooling in `tools/`
+
+## Conventions
+
+- All binary data in JSON is uppercase hex (e.g., `"0D00EFBEADDE"`).
+  Whitespace in hex strings is allowed and stripped on parse.
+- All multi-byte integers are little-endian unless explicitly noted otherwise.
+- Test vector IDs use the format `{category}-{NNN}` (e.g., `ack-001`, `kiss-hw-003`).
+- Spec sections are numbered `00-` through `20-` and use RFC 2119 keywords.
+- The C++ reference implementation at github.com/meshcore-dev/MeshCore is the
+  source of truth for protocol behavior.
+
+## Key Protocol Constants
+
+- MAX_PACKET_PAYLOAD: 184 bytes
+- MAX_PATH_SIZE: 64 bytes
+- CIPHER_MAC_SIZE: 2 bytes
+- PATH_HASH_SIZE: 1 byte (default)
+- PUB_KEY_SIZE: 32 bytes
+- SIGNATURE_SIZE: 64 bytes
+- MAX_TRANS_UNIT: 255 bytes
+
+## File Organization
+
+- `spec/NN-topic.md` — Spec sections, numbered for reading order
+- `corpus/schema/` — JSON Schema for test vector validation
+- `corpus/{layer}/{type}/` — Test vectors organized by protocol layer
+- `tools/` — Python validation and generation scripts
+
+## Working With Test Vectors
+
+Each JSON file validates against `corpus/schema/test-vector.schema.json`.
+Vector types: `encode_decode`, `decode_only`, `invalid`.