M3.20: arrowheads: hollow outlines, tip/extend alignment, one opacity for line and head

Hollow heads draw as rings (the open arrow as a chevron) and closed paths get
no heads. The stroke stops under each head, so it never shows through a hollow
head or past the tip; stroke.set {arrowAlign: "extend"|"tip"} chooses whether
the tip extends past the end point (default) or sits on it with the stroke
shortened. Line and heads share one paint box (a gradient runs on into the
head) and take the stroke and object opacity once, as one layer on the canvas
and one group in PDF. StrokeLayer.arrow_align is a new defaulted field, so
older files load unchanged. The Stroke panel's Align buttons get two generated
icons and are disabled when the stroke has no head.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

assets/icons/dc-arrow-extend.svg

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+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="butt" stroke-linejoin="miter"><path d="M2 12H14"/><path d="M13 7.5L21.5 12L13 16.5Z" fill="currentColor" stroke="none"/><path d="M14 4V20" stroke-width="1" stroke-dasharray="2 1.5"/></svg>

assets/icons/dc-arrow-tip.svg

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+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="butt" stroke-linejoin="miter"><path d="M2 12H10"/><path d="M9.5 7.5L18 12L9.5 16.5Z" fill="currentColor" stroke="none"/><path d="M18 4V20" stroke-width="1" stroke-dasharray="2 1.5"/></svg>