[NikitkaRa]

Summary

• An owned corpus slice in crates/pptx/tests/corpus_charts.rs round-trips a combo chart with chart-level and per-series data labels, a merged table with both diagonal borders, and text columns, shrink autofit, vertical text, shape autofit, and a WordArt warp with an adjust.
• Chart trendlines are now in the model and survive open, save, and reopen: type (exp, linear, log, movingAvg, poly, power), polynomial order, moving-average period, forward and backward forecast, intercept, and whether the equation and R² are shown. They are not drawn.
• A combo series on a secondary value axis is already read (Series.secondary_axis, y2_*). Writing that axis back is open PR Charts: every LibreOffice chart type, a data editor, and formatting for each #255, so this change does not rewrite chart axes.

Test plan

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test -p deckcraft-pptx --test corpus_charts
• cargo test -p deckcraft-pptx --lib trendlines_are_read
• cargo test --workspace with three known environment-only font/IME tests skipped (craft-fonts manifest missing on that machine): symbol_font_characters_draw_in_the_sym_font, korean_composes_in_the_notes_pane, cjk_text_in_the_interface_is_drawn_with_real_glyphs
• cargo xtask assets, cargo xtask layers, and cargo xtask wasm

[echelon]

Thank you for this contribution, @NikitkaRa! It has landed on main and will ship in the next DeckCraft release.

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