Shaper: an arrow widget switches a Shaper Group between Face Selection and Construction mode

A Shaper Group had no arrow widget for switching between Face Selection
and Construction mode.

With the Shaper or Selection tool active, a selected Shaper Group, or a
group with one of its faces selected, now shows an arrow beside the
middle of the right side of its source shapes' bounds. A click on it
isolates the source shapes (object.isolate). A second click leaves
isolation with the group selected (object.exitIsolation). The arrow
points right in Face Selection mode and left in Construction mode.

In Construction mode a Shaper click selects an original shape of the
isolated group. Before, it selected the whole group outside the
isolation, and shaper.select with source could pick an original of
another Shaper Group lying above it.

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

docs/shaper.md

@@ -11,8 +11,10 @@ Scribble over overlapping paths to make an editable Shaper Group:
 
 Click once to select the group; click again to select its visible fill and change its color. Double-click with Shaper to enter construction editing and select an original shape. The Selection tool then moves or resizes it normally, and the visible composition updates. Escape leaves construction editing. The original live shapes and edit recipes survive native save/reopen. Outside construction editing, selection highlights and bounds show only the visible result.
 
+While the group or one of its faces is selected (Face Selection mode) and the Shaper or Selection tool is active, an arrow widget sits beside the middle of the right side of the original shapes’ bounds. Click it to enter Construction mode: the original shapes are isolated and nothing is selected, and a click with Shaper selects the group’s original shape under the pointer. Click the widget again to leave Construction mode with the group selected. The arrow points right in Face Selection mode and left in Construction mode.
+
 Object › Shaper › Release restores the original paths. Expand keeps the visible result as ordinary paths. Each scribble and source edit records one undo step. Sources in a composition must share a parent layer/group; Shaper limits a composition to 64 source paths and 4,096 segments.
 
-The construction-mode arrow widget and finer gesture heuristics remain to be implemented.
+Finer gesture heuristics remain to be implemented.
 
 Behavior reference: [public Shaper documentation](https://helpx.adobe.com/illustrator/using/building-new-shapes-using-shape.html). Rotation steps and independent fill/stroke cases follow the contributor’s supplied requirements. No reference images or product assets are included.

docs/target-app-parity.md

@@ -456,7 +456,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - Shortcut sets other apps save as text (`.kys`) open in Import Shortcut Set from File › Open, from Edit › Keyboard Shortcuts › Import… in the desktop app, or from a drop on the window. It matches each entry to a tool, command or panel by name and decodes the file's key and modifier codes where VectorCraft's default shortcuts agree on them; the user answers the rest, and Apply fills the Keyboard Shortcuts dialog's working set. The answers are kept with the preferences for the next import (`shortcutSet.read`, `importMatches.*`).
   - On Windows and Linux the app bar is the window's title bar (its own minimize, maximize and close buttons, drag to move, edges to resize), with the dragon app icon as the brand mark; dialogs and menus size to their content. As on a native menu bar, once one menu is open, moving the pointer onto another title opens that one.
 - **Tools:**
-  - **Shaper:** uneven squares and rectangles become live rectangles even when drawing begins mid-edge. Recognition uses uniformly spaced samples and closed-loop corners, fits rectangle edges/angles rather than requiring a nearly perfect bounding-box fill, and includes the pointer-release position. Rectangles/squares and ellipses snap to 45° steps, hexagons to 90°, and triangles point up or down; lines keep their free angle even with Shift. Shaper Groups retain the live originals, merge regions with the gesture-origin fill, punch fills independently from strokes, remove boundaries on exiting scribbles and trim protruding stroke pieces. Source moves/resizes re-evaluate the result in the same undo step; double-click enters construction editing. Native save/reopen, face recoloring, Release and Expand are covered. The construction-mode widget and finer gesture heuristics remain.
+  - **Shaper:** uneven squares and rectangles become live rectangles even when drawing begins mid-edge. Recognition uses uniformly spaced samples and closed-loop corners, fits rectangle edges/angles rather than requiring a nearly perfect bounding-box fill, and includes the pointer-release position. Rectangles/squares and ellipses snap to 45° steps, hexagons to 90°, and triangles point up or down; lines keep their free angle even with Shift. Shaper Groups retain the live originals, merge regions with the gesture-origin fill, punch fills independently from strokes, remove boundaries on exiting scribbles and trim protruding stroke pieces. Source moves/resizes re-evaluate the result in the same undo step; double-click enters construction editing, and with the Shaper or Selection tool a click on the arrow widget beside a Shaper Group's original shapes switches the group between Face Selection and Construction mode. Native save/reopen, face recoloring, Release and Expand are covered. Finer gesture heuristics remain.
   - **Selection:** Selection, Direct/Group Selection, Magic Wand, Lasso. A Shift-drag marquee toggles what it reaches: objects with the Selection tool, anchors with Direct and Group Selection (#483); the Lasso adds with Shift and subtracts with Alt. Dragging a path segment with Direct Selection bends it when it's curved (its anchors stay) and moves its two anchors when it's straight, one undo step; the fill, Shift or Alt still pick the whole path (Alt-drag copies it).
     A key object needs another selected object outside it. A Shift-click or any other change that leaves one object selected clears the key, and `select.key` sets none on one object or on a group selected with only its own members. With one object selected, the Align panel, the Control bar and the Properties panel align it to the artboard whatever Align To is set to.
     With two or more objects selected and no key object, Align to Key Object makes the topmost of them the key, whatever order they were selected in, and the Align panel, the Control bar and the Properties panel align to it. Align To keeps its Selection or Artboard choice and shows it again once the key is cleared.