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>

CONTRIBUTING.md

@@ -0,0 +1,35 @@
+# Contributing to MeshCore Spec
+
+## Adding Test Vectors
+
+1. Choose the appropriate directory under `corpus/` based on protocol layer and
+   packet type.
+2. Add vectors to an existing JSON file, or create a new file if testing a
+   distinct aspect.
+3. Every JSON file must validate against `corpus/schema/test-vector.schema.json`.
+4. Use uppercase hex for binary data. Whitespace is allowed for readability.
+5. Each vector needs a unique `id` within its file and a clear `description`.
+6. For `encode_decode` vectors, verify both directions: encoding the structured
+   form produces the binary, and decoding the binary recovers the structure.
+
+## Editing the Specification
+
+1. Spec documents live in `spec/` and are numbered for reading order.
+2. Use RFC 2119 language (MUST, SHOULD, MAY) for normative requirements.
+3. Include ASCII wire format diagrams where applicable.
+4. Cross-reference related spec sections and test vector files.
+5. The C++ reference implementation is the source of truth — if the spec
+   disagrees with the firmware, the spec needs updating.
+
+## Generating Vectors from Reference Implementation
+
+See `tools/generate/README.md` for instructions on building and running the
+corpus generation harness against the C++ MeshCore library.
+
+## Validating the Corpus
+
+```bash
+python tools/validate.py
+```
+
+This checks all JSON files against the schema and verifies internal consistency.