Gradients: the Gradient panel's Method menu and paint.editGradient's method choose Classic or Perceptual

Illustrator 30.0's Method choice in the Gradient panel
(https://helpx.adobe.com/illustrator/desktop/paint-and-fill/create-and-edit-gradients/control-perceptual-interpolation-in-gradients.html).

- paint.editGradient takes `method: classic|perceptual` (one undo step);
  paint.setFill/setStroke take it in `gradient`, and gradient params (swatches,
  copied paints) carry it when Perceptual. Anything else is refused without
  editing. One edit can set `method` and `dither` together.
- The Gradient panel shows a Method menu (Classic, Perceptual) for linear and
  radial gradients, dimmed without a gradient; the Dither box stays under the
  stops.
- i18n: "Perceptual" where a complete catalog lacked it, and the `method`
  error in all 24 catalogs.
- Docs: G13 (estimate 14–24 h → 13–23 h, after Dither's 15–25 h → 14–24 h),
  target-app-parity (row and evidence), ui-parity, mcp.md, plugins.md.

crates/engine/src/cmd/gradient.rs

@@ -1,7 +1,7 @@
 //! The Gradient panel and the Gradient tool: in-place gradient edits and the gradient vector.
 
 use serde_json::{Value, json};
-use vectorcraft_color::{Gradient, GradientGeom, GradientKind, GradientPaint, GradientStop, Paint};
+use vectorcraft_color::{Gradient, GradientGeom, GradientKind, GradientMethod, GradientPaint, GradientStop, Paint};
 use vectorcraft_doc::appearance::stroke_paint_bounds;
 use vectorcraft_doc::{Document, Node, NodeKind, StrokeGradientMode};
 use vectorcraft_geom::{Affine, Point, Rect};
@@ -20,7 +20,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Gradient",
             ["Window", "Gradient"],
             None,
-            "{stroke?: bool (default: the targeted item's kind, else the active proxy), kind?: linear|radial|freeform (freeform places points on each object, coloured along the stops), mode?: points|lines (freeform: how the Gradient tool adds points), stops?: [{offset 0..1, color? (needed without swatch), opacity? 0..1 (or 0..100), midpoint? 0.13..0.87, swatch?: colour swatch name (a global or spot colour or tint swatch links the stop, so swatch edits recolour it and a spot stop prints on its plate; a process colour just gives its colour), tint?: 0..100 (% of the linked swatch; default 100, or a tint swatch's own)}] (at least 2; a freeform gradient's points are recoloured along them), keepModel?: bool (as paint.setFill; a stop color the gradient already has keeps its model), angle?: deg, aspect?: %, reverse?: bool, dither?: bool (fills only: a linear or radial gradient fill is drawn with fine noise that hides banding, and as pixels in SVG and PDF exports), item?: fill/stroke item index|null (omitted: the Appearance panel's active item when it is of the edited kind), ids?|id?, strokeMode?: within|along|across (strokes only: the gradient lies on the page and shows through the stroke, runs from the start of each subpath to its end, or runs from the stroke's left edge to its right all along it; with nothing selected, for the next object drawn; type characters' own strokes always paint within), target?: \"object\"|\"contents\"} edit the gradient in place (keeps its placement); solid/none paints become the default gradient",
+            "{stroke?: bool (default: the targeted item's kind, else the active proxy), kind?: linear|radial|freeform (freeform places points on each object, coloured along the stops), mode?: points|lines (freeform: how the Gradient tool adds points), method?: classic|perceptual (how the colours between the stops mix: in display RGB, or perceptually (OKLab) so the steps look even, which SVG and PDF exports draw as an image; freeform gradients mix their own way), stops?: [{offset 0..1, color? (needed without swatch), opacity? 0..1 (or 0..100), midpoint? 0.13..0.87, swatch?: colour swatch name (a global or spot colour or tint swatch links the stop, so swatch edits recolour it and a spot stop prints on its plate; a process colour just gives its colour), tint?: 0..100 (% of the linked swatch; default 100, or a tint swatch's own)}] (at least 2; a freeform gradient's points are recoloured along them), keepModel?: bool (as paint.setFill; a stop color the gradient already has keeps its model), angle?: deg, aspect?: %, reverse?: bool, dither?: bool (fills only: a linear or radial gradient fill is drawn with fine noise that hides banding, and as pixels in SVG and PDF exports), item?: fill/stroke item index|null (omitted: the Appearance panel's active item when it is of the edited kind), ids?|id?, strokeMode?: within|along|across (strokes only: the gradient lies on the page and shows through the stroke, runs from the start of each subpath to its end, or runs from the stroke's left edge to its right all along it; with nothing selected, for the next object drawn; type characters' own strokes always paint within), target?: \"object\"|\"contents\"} edit the gradient in place (keeps its placement); solid/none paints become the default gradient",
             has_doc,
             edit_gradient
         ),
@@ -105,6 +105,14 @@ fn parse_kind(k: &str) -> Parsed<GradientKind> {
     GradientKind::parse(k).ok_or_else(|| format!("unknown gradient kind `{k}` (linear, radial, freeform)"))
 }
 
+/// The `method` param of `p`, if given: how the colours between the stops mix.
+fn method_param(p: &Value) -> Parsed<Option<GradientMethod>> {
+    match p.get("method") {
+        None | Some(Value::Null) => Ok(None),
+        Some(v) => v.as_str().and_then(GradientMethod::parse).map(Some).ok_or_else(|| "`method` must be classic or perceptual".into()),
+    }
+}
+
 /// `v` (params holding `stops`) with each stop that names a `swatch` resolved through `d`
 /// ([`swatch_solid`]): a global or spot colour or a tint swatch gives the stop its colour, link and
 /// tint, a process colour just its colour. A stop linked to a swatch that no longer exists keeps
@@ -229,6 +237,7 @@ pub(crate) fn parse_gradient(g: &Value) -> Parsed<GradientPaint> {
         None | Some(Value::Null) => false,
         Some(v) => v.as_bool().ok_or("`dither` must be true or false")?,
     };
+    gp.gradient.method = method_param(g)?.unwrap_or_default();
     gp.angle = f64_or(g, "angle", 0.0);
     gp.swatch = str_param(g, "swatch").map(str::to_string);
     let geom = GeomEdit::parse(g)?;
@@ -326,6 +335,9 @@ pub(crate) fn apply_gradient_edit_in(
     if let Some(d) = p.get("dither").and_then(Value::as_bool) {
         gp.gradient.dither = d;
     }
+    if let Some(m) = method_param(p)? {
+        gp.gradient.method = m;
+    }
     // Angle and aspect edits keep the focal point in its place in the extent ellipse.
     let before = gp.geom;
     if let Some(a) = p.get("angle").and_then(Value::as_f64) {