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>

.gitignore

@@ -0,0 +1,22 @@
+# Python
+__pycache__/
+*.py[cod]
+*.egg-info/
+.venv/
+venv/
+
+# Editors
+*.swp
+*.swo
+*~
+.vscode/
+.idea/
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Build artifacts
+/build/
+/dist/
+/references/