The export model (`pdfcraft_export::Block`/`Cell`) had no colour, so
`export_pages` dropped the fill colour that `edit::TextBlock` already
carries and every run came out black.
Block and Cell now carry an sRGB colour, mapped from TextBlock.color.
Word runs get `<w:color w:val="RRGGBB"/>`, HTML paragraphs, headings and
cells a `style="color:#RRGGBB"`, and RTF a colour table with `\cfN`.
Black text keeps each format's default; NaN and out-of-range components
clamp.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
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