Artboard tool: '+' buttons add a board beside the selected one; ⇧ keeps proportions (#3033) (#3046)

* Tools: Artboard tool (#2858)

Photoshop's Artboard tool in the Move flyout (V, ⇧V cycles Move ↔ Artboard):
dragging on an empty spot draws a new board (`layer.new.artboard`), clicking a
board selects it, dragging it moves it with its contents, and the selected
board's edge and corner handles resize it (`layer.artboard.set`,
moveContents: false). Each gesture is one undo step. Options bar: Size preset,
W/H, Make Portrait, Make Landscape, Add New Artboard. Original toolbar glyph
(assets/icons/artboard.svg, ATTRIBUTION.md row).

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

* Artboard tool: '+' buttons add a board beside the selected one; ⇧ keeps proportions (#3033)

With the Artboard tool, the selected board shows a "+" button beside each
side. A click adds an empty board of the same size (background and preset
copied) on that side, the 100 px artboard gap away and past any board in
the way; ⌥-click adds a copy with its contents instead. A board left of or
above the canvas origin shifts the whole document right/down and grows the
canvas, so it stays visible. Both go through the new engine command
`layer.artboard.addBeside` {layer?, side, duplicate} (one undo step).

⇧ while dragging a corner handle keeps the board's aspect ratio: the
opposite corner stays and the board scales by the larger axis factor.

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

---------

Co-authored-by: Claude Opus 5.5 <[email protected]>

scorecard/file_compat.toml

@@ -143,7 +143,7 @@ id = "FILE-215-20"
 target = "Artboard tool with handles and + buttons, Duplicate Artboard beside, vector PDF pages"
 status = "partial"
 issue = 215
-note = "artboard_tool.rs: Artboard tool draws, selects, moves and resizes boards by their handles; Duplicate Artboard places the copy beside (#1584); no + buttons, raster PDF pages only"
+note = "artboard_tool.rs: Artboard tool draws, selects, moves and resizes boards by their handles (⇧ keeps proportions), its + buttons add a board or (⌥) a copy beside the selected one (#3033); Duplicate Artboard places the copy beside (#1584); raster PDF pages only"
 
 [[item]]
 id = "FILE-215-21"