Shape Builder: Pick Color From Artwork takes the look where the drag starts, else where it ends

With Pick Color From Artwork, a Shape Builder merge took the look of
the front-most object over the first covered region the drag touched,
and the current fill when the drag touched only areas no object
covers. The reference app's combine page gives the look of the object
where the drag starts, else of the object where it ends, else of the
topmost selected object.

shapeBuilder.merge now follows that rule: the merged shape takes the
appearance, opacity and blending mode of the front-most of the objects
it is given (the selection by default) under the first point, else of
the one under the last point, else of the front-most of those objects.
A drag from a hole across a gray bar into a red square now gives a red
shape, where it gave the bar's look. A click in an area no selected
object covers, such as a hole that gray bars enclose or a triangle
that lines enclose, takes the front-most selected object's look
instead of the current fill. Pick Color From Color Swatches works as
before. The tool already sends the press point first and the release
point last.

Docs: docs/mcp.md, docs/target-app-parity.md.

crates/tools/src/builder.rs

@@ -495,7 +495,8 @@ pub struct ShapeBuilderOptions {
     /// Pick Color From Color Swatches (`pickColorFrom: "swatches"`; off: `"artwork"`): a merge
     /// fills the new shape with the current fill, and the Left and Right arrow keys step the current
     /// fill through the document's color swatches. With Artwork, the shape takes the look of the
-    /// art it is merged from.
+    /// object where the drag starts, else of the one where it ends, else of the front-most selected
+    /// object.
     pub swatches: bool,
     /// Cursor Swatch Preview (`cursorSwatchPreview`, off): with Color Swatches, the current fill
     /// and the swatches before and after it show as chips above the pointer while no drag is on.