M3.62: thirty more generated arrowheads with rendered previews

Arrowhead gains 30 original shapes generated in code, appended after the first
ten so saved names stay stable: Barbed, Half Arrow (left/right), Concave,
Double Arrow, Chevron, Feather, Swallowtail, Reverse Triangle, Triangle to Bar,
Kite, Leaf, Drop, Half Circle, Oval, Target, Tag (+ open), Diamond (open),
Hexagon (+ open), Star, Cross, Plus, Double Bar, Dot on Bar, Slash, Double
Slash, Bracket and Fork. Each is drawn in local units of the head's weight
inside the usual 4 x 4 box, with its own stop for the stroke under it.
Arrowhead::label() names them in the menus. The Stroke panel's arrowhead
dropdowns render every head as the canvas draws it (cached textures, a shared
widgets::doc_preview that the Brushes panel now uses too) in a scrolling list.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/stroke.rs

@@ -18,7 +18,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Stroke Options",
             ["Window", "Stroke"],
             None,
-            "{weight?: pt, cap?: butt|round|square, join?: miter|round|bevel, miterLimit?, align?: center|inside|outside, dash?: [d,g,…]|null (a 0 dash with a round or projecting cap draws dots or squares; a new pattern keeps the current offset and alignment), dashOffset? (exact dashes only), alignDashes?: bool (true: dashes fitted to corners and path ends, every run between them holding whole periods with a dash centred on each corner and end; false, the default for a new pattern: exact lengths), startArrow?, endArrow?: Arrow|ArrowOpen|Triangle|TriangleOpen|Circle|CircleOpen|Square|SquareOpen|Diamond|Bar|null, arrowAlign?: \"extend\" (tip past the end point, default)|\"tip\" (tip on the end point; the stroke is shortened), profile?: \"uniform\"|\"lens\"|\"taperStart\"|\"taperEnd\"|\"pinch\"|\"teardrop\"|\"wave\"|a saved profile's name (stroke.widthProfile.list), item?: stroke item index|null (omitted: the Appearance panel's active item if it is a stroke, else the top stroke, created when missing), ids?} Without a stroke item, type takes weight, cap, join, miterLimit and the dash options as its characters' stroke (every run; text.setRangeStyle `strokeOptions` styles a range), and images and symbol instances (also in groups) are left alone",
+            "{weight?: pt, cap?: butt|round|square, join?: miter|round|bevel, miterLimit?, align?: center|inside|outside, dash?: [d,g,…]|null (a 0 dash with a round or projecting cap draws dots or squares; a new pattern keeps the current offset and alignment), dashOffset? (exact dashes only), alignDashes?: bool (true: dashes fitted to corners and path ends, every run between them holding whole periods with a dash centred on each corner and end; false, the default for a new pattern: exact lengths), startArrow?, endArrow?: Arrow|ArrowOpen|Barbed|Concave|DoubleArrow|HalfArrowLeft|HalfArrowRight|Chevron|Feather|Swallowtail|Triangle|TriangleOpen|TriangleReverse|TriangleBar|Kite|Leaf|Drop|Circle|CircleOpen|HalfCircle|Oval|Target|Square|SquareOpen|Tag|TagOpen|Diamond|DiamondOpen|Hexagon|HexagonOpen|Star|Cross|Plus|Bar|DoubleBar|DotOnBar|Slash|DoubleSlash|Bracket|Fork|null (HalfArrowLeft/Right: one barb, left or right of the direction the head points), arrowAlign?: \"extend\" (tip past the end point, default)|\"tip\" (tip on the end point; the stroke is shortened), profile?: \"uniform\"|\"lens\"|\"taperStart\"|\"taperEnd\"|\"pinch\"|\"teardrop\"|\"wave\"|a saved profile's name (stroke.widthProfile.list), item?: stroke item index|null (omitted: the Appearance panel's active item if it is a stroke, else the top stroke, created when missing), ids?} Without a stroke item, type takes weight, cap, join, miterLimit and the dash options as its characters' stroke (every run; text.setRangeStyle `strokeOptions` styles a range), and images and symbol instances (also in groups) are left alone",
             has_doc,
             stroke_set
         ),