Closes #9326.

Adds a rectangle mode to the three polyline insertion tools. Press R while the tool is running to toggle it, then pick two opposite corners — the second pick completes the shape, so a rectangular slab is two clicks instead of four plus a confirmation.

The status bar shows Rectangle: ON/OFF, the mode is remembered between invocations so a series of rectangles doesn't need R every time, Backspace restarts the rectangle and Esc cancels the tool. Typed input still works: the distance/angle and X/Y inputs place the opposite corner, so an exact rectangle can be typed rather than drawn.

Implementation notes

The provisional corners are written into insertion_polyline itself, with the first corner repeated at the end so the shape reads as closed. That is what lets the existing ProductDecorator preview, the per-side dimension labels and the total length keep working without any decorator changes, and it means walls and profiles are generated and joined by the same code path as closing a polyline by hand.

Because the polyline then holds more than one point while the second corner is still being picked, calculate_distance_and_angle and calculate_x_y_and_z skip their "second to last point" reference when rectangle_mode is set, so typed distance and angle stay relative to the first corner.

Two details worth a look during review:

• The rectangle spans the two axes of the tool's active plane, defaulting to a plane parallel to XY. The profile tool can draw on XZ and YZ and can pick points anywhere in space, so the coordinate of the axis the rectangle doesn't span always comes from the first corner, keeping the shape planar. It also follows the active custom transform orientation, so rectangles can be drawn square to a rotated grid.
• Increment snapping rounds the two sides of the rectangle rather than the diagonal running to the cursor. Rounding the diagonal produces sizes like 3.13 x 4.79, which defeats the purpose.

The shared modal flow lives in PolylineOperator.handle_rectangle_drawing, which finishes the shape through a finish method each of the three operators implements. Each modal gained three lines: the toggle, the instruction and status entries, and the handler call.

Testing

• 7 new cases in test/tool/test_polyline.py covering the corner math, the XZ/YZ planes, the planarity rule, snapping the sides, and rebuilding the preview in place (12 pass in the file).
• Three tests in the event-simulating modal harness, make test-modal MODULE=rectangle, driving each of the three tools through real events: slab footprint rectangular, 4 walls at 4 corners, 4 coplanar beams.