ci+docs: add canary CI signal and release checklist

[tmustier]

Summary

Stabilization pass focused on release visibility and user-facing docs:

1. Add an explicit CI canary signal
2. Sync chart/schema docs with current shipped behavior
3. Add a release checklist for repeatable gates

Changes

CI

• .github/workflows/ci.yml
  • added a dedicated canary job (Python 3.12)
  • runs only: pytest -q tests/test_canary_decks.py

This makes behavior-level deck health visible as a first-class CI status check,
separate from the full test matrix.

Documentation

• docs/CHARTS.md

  • clarified current JSON chart usage
  • documented bar template copy options (chart_template_copy, related fields)
  • documented connector options (connector_style, connector_dash_style, connector_value)

• docs/CHART-CELLS.md

  • updated status to regression/canary-covered
  • added current behavior highlights (manual horizontal label offsets, waterfall connectors, sub styling)
  • replaced stale implementation-plan / outdated out-of-scope section with current implementation notes and known limits

• docs/INPUT-SCHEMA.md

  • documented sub usage in header/cell-rich objects
  • documented sub render semantics (new line, non-bold, body color)
  • clarified row-group header object support ({text, sub})

• docs/RELEASE-CHECKLIST.md (new)

  • codifies local, canary, CI, manual-openability, and docs sanity gates

• README.md

  • linked release checklist from Development section

Validation

• .venv/bin/ruff check clean_slides tests
• .venv/bin/pyright
• .venv/bin/pytest -q
• .venv/bin/pre-commit run --all-files

All green locally.