Robustness fixes: logging, dependency, packaging, chart batching

[npogeant]

Summary

• Replace internal diagnostic print() calls with the logging module across content extraction, formatters, updaters, and elements. The intentional user-facing display output (SlidePrinter/TablePrinter, used by list_content()/show_data()) is untouched — those are meant to print.
• Declare Pillow as an explicit dependency: adders/image_adder.py imports PIL directly, but it was only ever satisfied transitively through python-pptx. Pinning it directly avoids a future break if that transitive dependency ever changes.
• Add a py.typed marker (and the package-data entry needed to ship it in the wheel) so downstream consumers' type checkers actually pick up this package's type hints. Verified the built wheel includes it.
• Batch chart updates: Deck.update_chart previously called chart.update_series() per series (each doing a full replace_data) and then chart.save_changes() again — N+1 full chart XML rewrites for N series. Added DeckChart.update() to merge categories/series and apply once; Deck.update_chart now uses it. update_series/update_categories are unchanged for direct/individual use.
• De-duplicate remove_text/remove_chart/remove_table in DeckSlide, which were copy-pasted apart from the getter and error label, into one _remove_element helper. Error messages are unchanged.

No version bump — this is not a release yet.

Test plan

• pytest -q — 33 passed
• mypy --ignore-missing-imports src/deckflow — no issues
• Built the wheel and confirmed py.typed and the Pillow requirement are present in the artifact