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.

docs/plugins.md

@@ -198,7 +198,7 @@ type and `blob` but without their data, which follows the JSON in a saved file;
   `paint`, `width` in points, `join` (`"miter"`, `"round"` or `"bevel"`), `cap` (`"butt"`, `"round"` or `"square"`)
   and `miterLimit`. Paints use the
   native file format's form: `{"type": "solid", "color", "swatch"?, "tint"?}`, `{"type": "gradient", "gradient":
-  {"kind": "Linear"|"Radial"|"Freeform", "stops": [{offset, color, opacity, midpoint}], "dither"?}, "geom"?, "angle",
+  {"kind": "Linear"|"Radial"|"Freeform", "stops": [{offset, color, opacity, midpoint}], "dither"?, "method"?: "Perceptual"}, "geom"?, "angle",
   "freeform"?}`, `{"type": "pattern", "pattern", "xf"?}`. Colours are `{"model": "rgb", r, g, b}`, `{"model": "cmyk",
   c, m, y, k}`, `{"model": "gray", k}` (ink: 0 white, 1 black) or `{"model": "lab", l, a, b}`; components 0–1 except
   Lab's.