Function computeIndicatorCommand

Build the visible-window indicator DrawCommand for the minimap.

The main canvas's visible world rect is:

worldX = mainView.x
worldY = mainView.y
worldW = mainViewDims.width  / mainView.scale.x
worldH = mainViewDims.height / mainView.scale.y
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That world rect is then transformed into the minimap's screen coordinates through minimapView.

The returned command is a dashed-stroke path rect with no fill — matching the ViewportLayerDemo indicator math.