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/cmd/gradient.rs

@@ -124,6 +124,8 @@ pub(crate) fn parse_stops(v: &Value) -> Parsed<Vec<GradientStop>> {
                 color,
                 opacity: opacity.clamp(0.0, 1.0) as f32,
                 midpoint: num("midpoint")?.unwrap_or(0.5).clamp(0.13, 0.87) as f32,
+                swatch: None,
+                tint: 1.0,
             })
         })
         .collect::<Parsed<Vec<_>>>()?;