Stroke: custom arrowheads from an arrowheads file join the arrowhead menus

The Stroke panel menu's Edit Arrowheads… (stroke.arrowheads.edit) opens
Arrowheads.vectorcraft next to the preferences, creating it with one
example head when it is missing. Each symbol in the file is one head,
listed by name after the built-in heads in both arrowhead menus: it
points right, its registration point is the tip, and its height is drawn
as wide as the built-in heads. The file is read at startup and again
each time VectorCraft saves it; stroke.arrowheads.reload and
stroke.arrowheads.list read and list it.

Arrowhead gets a Custom variant, appended after the built-in names, that
carries the head's name and outline, so a document that uses one draws,
saves, reopens and exports it without the arrowheads file. stroke.set
takes {"custom": name} or the {"Custom": {name, path}} form that
document.inspect reports.

Part of #5421.

docs/ui-parity.md

@@ -406,6 +406,7 @@ next unit in the order a right-click on a ruler lists them, from Feet back to Po
 | Select › Start Global Edit / Stop Global Edit; Properties › Quick Actions with the Global Edit Options popover (Match: Appearance, Size; On Artboards: All Artboards or a range, Include Objects On Canvas; Result(s)); Escape stops it; the matched objects are highlighted in their layer colour while it runs; a Shift-click on a highlighted match leaves it out | partial | #3521, `select.globalEdit` / `select.globalEditOptions` / `select.globalEditExclude`, `crates/engine/src/tests/globaledit.rs`; paths, compound paths, symbol instances and groups of them (similar groups, #4939: edits inside one group reach the same object in the others; [public source](https://helpx.adobe.com/illustrator/desktop/manage-objects/edit-objects/edit-groups-with-similar-objects.html)); Shift-click to leave a match out ([public source](https://helpx.adobe.com/ph_fil/illustrator/how-to/global-editing-sensei.html)); the Control bar's Global Edit Options dropdown opens the same options; rotated copies and highlighting matches before the start remain ([public source](https://helpx.adobe.com/illustrator/desktop/manage-objects/edit-objects/edit-similar-objects.html)) |
 | Contextual task bar | done | |
 | Control bar with Transform and Stroke popovers | done | |
+| Stroke panel: custom arrowheads after the built-in ones in both arrowhead menus, from the arrowheads file that the panel menu's Edit Arrowheads… opens; saving the file reloads the menus, and a stroke keeps a copy of its custom head's art | done | [Apply stroke on an object](https://helpx.adobe.com/illustrator/using/stroke-object.html) (Customize arrowheads); the file's layout (one symbol per head, pointing right, tip at the registration point, height drawn as wide as the built-in heads) is VectorCraft's own; `stroke.arrowheads.edit`, `stroke.arrowheads.list`, `stroke.arrowheads.reload`, `stroke.set` `{custom: name}`; engine `tests::arrowheads`, `stroke::tests_arrows::custom_heads_draw_their_own_art_at_the_tip` |
 | Control bar: Paragraph: link and Align left/center/right while type is selected (#5372) | done | After the Character controls, as the public documentation has the Control panel format paragraphs ([Paragraph panel overview](https://helpx.adobe.com/illustrator/using/formatting-paragraphs.html), [Align text](https://helpx.adobe.com/illustrator/using/align-text.html)); the link opens the Paragraph panel in a popover, as Character: does with the Character panel. The bar trims from its right end: X/Y/W/H, Distribute, Align, area type vertical alignment, then these buttons, then the link. The three-button set is our choice for room; all seven alignments are in the popover. `panels/paragraph/tests.rs` headless tests (`the_control_bar_shows_the_paragraph_alignment_of_the_selected_type`, `the_control_bar_trims_its_paragraph_section_after_the_controls_to_its_right`) |
 | Pathfinder shape results keep their source names and SVG IDs | done | #1293; Minus Front keeps the back name, Minus Back the front name; Unite/Intersect/Exclude use the backmost named input; compound output carries it once; geometry and paint unchanged |
 | Attributes fill-rule controls with a compound member selected | done | Edits the compound's rule; direction still edits only the member; rendered-hole and Undo/Redo regression tests (#1197) |