Render: a moved path reuses its gradient stroke slices

A path whose stroke has a gradient along or across it kept its slices under
the node's address, so every nudge made the slices again for the whole path,
although its stroke outline was already reused. The renderer now keeps the key
of a stroke's latest slices under the object's id, the stroke's place and the
tolerance level, and when the object has only moved (same stroke and gradient,
same fill rule, anchors moved by one offset) it draws those slices moved by the
offset, translating only the slices that reach the piece being clipped. Scale,
rotate, an anchor edit or a change to the stroke or its gradient makes the
slices again. The pixels are the same.

docs/target-app-parity.md

@@ -1280,6 +1280,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   The renderer keeps the path of a path or compound path of 1,024 anchors or more by object, with the bounds of its segments and of its miter spikes in runs of 128 segments. When anchors move and the subpaths and anchor counts stay the same, the next frame changes only the segments that start or end at a moved anchor and measures again only the runs that hold them or the segments with length before them, instead of making the whole path again and measuring it whole for culling. The pixels and the cull bounds are the same. On Windows at 1600 × 1200, the frame after one anchor moves takes 8.2 ms instead of 30.1 ms with an open path of 200,000 anchors, 4.3 ms instead of 10.4 ms with a closed path of 50,000 anchors, and 6.6 ms instead of 11.9 ms with the filled closed path of 50,000 anchors and a dashed stroke (medians of five runs of `anchor_bench`).
   When a path object moves without changing shape, the renderer draws its stroke from the outline it made before the move, moved by the same offset. It outlines the stroke again when the path is scaled, rotated or edited, when the stroke changes, and when the move takes the view past the part of a long path it outlined. The pixels are the same. At 1600 × 1200, the frame after a 1 pt nudge takes 123 ms instead of 403 ms with an open path of 200,000 anchors in view (103 ms with nothing changed), and 13 ms instead of 26 ms with a closed path of 50,000 anchors whose stroke is dashed and has a width profile.
   When a path object moves without changing shape, the renderer draws its stroke from the outline it made before the move, moved by the same offset. It outlines the stroke again when the path is scaled, rotated or edited, when the stroke changes, and when the move takes the view past the part of a long path it outlined. The pixels are the same. At 1600 × 1200, the frame after a 1 pt nudge takes 128 ms instead of 405 ms with an open path of 200,000 anchors in view (104 ms with nothing changed), and 14 ms instead of 28 ms with a closed path of 50,000 anchors whose stroke is dashed and has a width profile.
+  When the stroke of a path moved this way has a gradient along or across it, the renderer also draws the gradient from the slices it made before the move, moved by the same offset. It makes the slices again when the path is scaled, rotated or edited, and when the stroke or its gradient changes. The pixels are the same. On Windows at 1600 × 1000, the frame after a 1 pt nudge of an open path of 200,000 anchors takes 75 ms instead of 1,237 ms with the gradient along the stroke and 69 ms instead of 1,420 ms with it across, and with an open path of 50,000 anchors 22 ms instead of 313 ms and 21 ms instead of 352 ms (medians of five interleaved runs).
   The renderer keeps the path of a path or compound path of 1,024 anchors or more by object, with the bounds of its segments and of its miter spikes in runs of 128 segments. When anchors move and the subpaths and anchor counts stay the same, the next frame changes only the segments that start or end at a moved anchor and measures again only the runs that hold them or the segments with length before them, instead of making the whole path again and measuring it whole for culling. The pixels and the cull bounds are the same. On Windows at 1600 × 1200, the frame after one anchor moves takes 8.2 ms instead of 30.1 ms with an open path of 200,000 anchors, 4.3 ms instead of 10.4 ms with a closed path of 50,000 anchors, and 6.6 ms instead of 11.9 ms with the filled closed path of 50,000 anchors and a dashed stroke (medians of five runs of `anchor_bench`).
   Type › Show Hidden Characters keeps the marks it places on each text object from one frame to the next. The canvas lays a text object out again when an edit changes it or its threaded story, or when the last frame that showed the marks did not draw it, and it lays out all type in view again when the fonts or the hyphenation exceptions change. With 1,000 two-line labels in view, drawing the marks takes 0.8 ms per frame in a release build instead of 19.7 ms.
   A selection with more than 100,000 points to outline shows its bounds instead of each object's outline and anchors. Paths count their anchors. Area type and type on a path count the anchors of their frame or path, and point type counts six: its box's corners and the ends of its baseline marker. Images, symbol instances, placed documents, and area type or type on a path in perspective count the four corners of their box, and gradient meshes and envelopes count their points and the ends of their mesh lines. The hover highlight leaves out an object past that count, and such an object shows its bounds as the key object. With 32,768 type objects, images, symbol instances, gradient meshes or envelopes selected, the selection's outlines and anchors add fewer than 100 vertices to the canvas's mesh (#2460).