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>

docs/control-protocol.md

@@ -76,7 +76,7 @@ as `ui.color_guide` (`variation`, `steps`, `amount`). Save Colors as Swatches ru
 (one swatch per colour, one undo step).
 
 Edit Colors dialogs: `ui.colorBalanceDialog` opens Adjust Colors (`colorBalance`: fields `mode` `gray`/`rgb`/`cmyk`/
-`global`, the channels `r` `g` `b` / `c` `m` `y` `k` / `gray` from −100 to 100, `convert`, `fill`, `stroke`,
+`global`, the channels `r` `g` `b` / `c` `m` `y` `k` / `gray` / `tint` (global mode) from −100 to 100, `convert`, `fill`, `stroke`,
 `preview`) and `ui.saturateDialog` opens Saturate (`saturate`: `intensity` −100..100, `preview`). Both preview on the
 canvas while open; `ui.dialog.confirm` keeps the result as one undo step (`edit.colors.adjustBalance` /
 `edit.colors.saturate`) and `ui.dialog.cancel` rolls it back. Global mode answers with an error until tints of global