#983: supersample Art Optimized edges in exports

Asset Export, File › Export, Save for Web and Object › Rasterize drew
each shape with its own smoothed edge, so where two shapes met edge to
edge (Divide, Shape Builder, abutting art) both half-covered the pixels
on the line and the background showed through as a thin ghost border.

With Art Optimized (the default) these now supersample, as the option's
name in Illustrator says: the art is drawn with hard edges at four times
the size (three or two times for large images, within a 40-megapixel
budget and the renderer's 65,535-pixel side) and each block averaged
into one pixel. Pixels shared by two shapes are fully owned by one or
the other at the finer size, so no seam is left, and the outer edges
are still smooth. The canvas keeps shape-by-shape smoothing for speed;
None and Type Optimized are unchanged.

RenderOptions::supersample turns it on (exports and Rasterize set it).
A render test checks two triangles sharing a diagonal: a seam with
shape-by-shape smoothing, none supersampled, and the factor's limits.

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

docs/gaps.md

@@ -86,8 +86,7 @@ from G14 and the budget run of G15: **~170–270 h** one agent, ~50–80 h wall
 
 ## G11 Correctness bugs in core tools
 
-- **Missing:** Shape Builder misses planar regions and targets whole shapes (#937, #893); Asset Export
-  anti-aliasing ghost borders (#983); Transform effect dialog's invisible checkboxes and missing options (#885);
+- **Missing:** Shape Builder misses planar regions and targets whole shapes (#937, #893); Transform effect dialog's invisible checkboxes and missing options (#885);
   PDF text boxes moving on open (#722); SVG units reverting to points (#864); bezier drag preview freezing (#834).
 - **Impact:** wrong output in everyday work.
 - **Estimate:** 8–15 h.