M3.59: tints of global and spot colours, tint swatches and Global colour balance

A colour linked to a global or spot swatch keeps a tint (Paint::Solid.tint, 1 by default and
left out of files at 100 %): paint.setFill/setStroke {swatch, tint} apply the swatch at that
percentage, swatch edits recolour every linked colour at its own tint, and Color::tinted
resolves it (CMYK and Gray inks scale, RGB mixes toward white). swatch.new saves a tint as a
tint swatch ("Ink 40%") linked to its base. Spot tints separate as that percentage of their
plate (Separations Preview, PDF Separation value) instead of an ink-ratio guess. Adjust Color
Balance's Global mode (tint) shifts the tints of linked colours. The Color panel shows Tint
mode (the swatch chip, its name and a T slider) for linked colours; the Swatches panel
highlights the tint swatch in use. Gradient stops get the same link and tint fields (the
swatch walks cover them), so GradientStop is no longer Copy.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/lib.rs

@@ -847,4 +847,6 @@ mod tests_textedit;
 #[cfg(test)]
 mod tests_tileedge;
 #[cfg(test)]
+mod tests_tints;
+#[cfg(test)]
 mod tests_xform;