test(canary): add CI deck render canary suite

[tmustier]

Summary

Add a dedicated render-canary suite that exercises representative, user-visible deck generation paths in CI.

What this adds

New file: tests/test_canary_decks.py

The suite covers 4 canary scenarios end-to-end:

1. Mixed YAML deck generation (cmd_generate)
   • metadata-only + table slide in one deck
   • validates slide count, openability, and expected text tokens
2. Waterfall chart-cells generation (cmd_generate)
   • horizontal waterfall chart-cells with connectors
   • validates chart type, connector presence, and rendered numeric labels
3. Multi-chart JSON deck generation (cmd_charts)
   • clustered horizontal + waterfall horizontal overlays in one deck
   • validates chart types, expected overlay text, and connector-dot segment footprint
4. Template-copy roundtrip through CLI path (cmd_charts)
   • validates chart_template_copy behavior by asserting generated
     ppt/charts/chart1.xml and chart1.xml.rels exactly match template parts

Why

This is a stabilization-focused canary pass (behavior-level), not internal refactor coverage.
It catches regressions in the exact paths users care about:

• YAML deck builds
• chart-cells + overlays
• JSON chart decks
• template-copy packaging correctness

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.