feat: version-agnostic analysis scripts with shared version helpers

- scripts/bicorder_common.py (new): single source of truth for historical
  gradient renames (COLUMN_RENAMES), version detection (bicorder_version col
  → version col → data/<type>_<version>/ dir convention), and training-CSV
  auto-selection (find_training_csv)
- classify_readings.py: auto-select training run by recorded bicorder version
  (excludes the input itself to avoid circular training), canonicalize old
  column names, version-mismatch warnings
- bicorder_classifier.py / export_model_for_js.py: share renames + dimension
  loader; instructive error when clustering results are missing;
  bicorder_version recorded in exported models
- compare_analyses.py: CLI (reference + comparison CSVs), rename
  canonicalization so runs of different versions align on shared gradients,
  Descriptor dedup (was silently skewing merges); legacy no-arg audit intact
- scripts/univariate_analysis.py (new): per-protocol/per-gradient averages,
  distributions, summary stats; --img publishes the three README summary
  charts to img/
- sync_readings.sh: defer classifier training to auto-matching; gitignore
  analysis/.venv and __pycache__

.gitignore

@@ -1,3 +1,5 @@
 tmp
 analysis/venv
+analysis/.venv
+__pycache__/
 .aider*