Artboards: Artboard Options' Show Center Mark and Show Cross Hairs draw on the canvas

The Artboard Options help pages describe two display options
(https://helpx.adobe.com/illustrator/desktop/create-manage-artboards/organize-manage-artboards/modify-display-settings-of-artboards.html,
.../set-video-display-options-for-artboards.html): a mark at the artboard's
center, and lines from the middle of each side through the center. The
document already had show_center_mark and show_cross_hairs on Artboard, but
nothing set or drew them.

artboard.setProps (each `artboards` entry too) and artboard.new take
showCenterMark and showCrossHairs as true/false, in the same single undo
step as the other fields; a locked artboard can still change them.
Artboard Options has both checkboxes, filled in from the artboard whether
it opens from the panel or a double-click with the Artboard tool, and its
New Artboard form passes them to artboard.new. The canvas draws them in the
new theme token artboard_mark, 1 point wide on screen at any zoom: a cross
12 points across at the center whose arms follow the artboard's sides, and
the two cross hairs. They are screen-only like the edges; exports and print
never read the fields. inspect_document reports them when on. Saved files
are unchanged: the two keys were already always written, so a document that
never shows them saves byte for byte as before, and a file without the keys
opens with both off. Video safe areas, video rulers and the pixel aspect
ratio are not part of this change.

Verification: artboard_display_marks_are_set_and_cleared_in_one_undo_step,
artboard_display_marks_round_trip_through_the_native_format,
artboard_display_marks_stay_out_of_exports (SVG and PDF identical with the
marks on and off), an_artboard_shows_its_center_mark_and_cross_hairs
(positions, screen-constant size under zoom, rotated view) and
artboard_options_show_center_mark_and_cross_hairs. cargo xtask ci: all 10
steps passed.

docs/mcp.md

@@ -3000,7 +3000,7 @@ value: true}`; the Control bar's and Properties' Scale Artwork with Artboard che
 Artboard) makes its handle drags proportional and passes `scaleArt`, as do Properties' and the Transform panel's
 artboard size fields while the tool is in use.
 
-`artboard.setProps {artboards: [{index, name?, color?, locked?, x?, y?, width?, height?}…], scaleArt?, moveArt?, …}` sets several
+`artboard.setProps {artboards: [{index, name?, color?, locked?, showCenterMark?, showCrossHairs?, x?, y?, width?, height?}…], scaleArt?, moveArt?, …}` sets several
 artboards in one undo step, each with its own fields; the other params apply to all of them. The art that moves or
 scales with each artboard is the art inside it before any of them changes, and an object inside several of them goes
 with the first one listed that moves or scales its art. An empty list and a list that names an artboard twice are
@@ -3052,6 +3052,13 @@ artboard × paper / 255; artboards without a color show the paper color. The tra
 it shows. Copies, duplicates and the native format keep the color; exports and print do not draw it. `inspect_document`
 lists it as the artboard's `color` (`#rrggbb`), and `ui.colorPicker {artboard: index}` opens the Color Picker for it.
 
+Artboard Options' Show Center Mark and Show Cross Hairs: `artboard.setProps {index, showCenterMark?: bool,
+showCrossHairs?: bool}` (or each entry of `artboards`) turns them on or off for that artboard in one undo step, and
+`artboard.new` takes them too. The canvas draws the center mark as a small cross at the artboard's center and the cross
+hairs as lines from the middle of each side through the center, 1 point wide on screen at any zoom; exports and print
+leave them out. Duplicates, copies and the native format keep them, and `inspect_document` adds `showCenterMark: true`
+and `showCrossHairs: true` to an artboard that shows them.
+
 ```json
 {"name":"run_command","arguments":{"command":"artboard.copy","params":{"index":0}}}
 {"name":"run_command","arguments":{"command":"edit.paste","params":{}}}