Render: the ink planes of CMYK documents with transparency are composed on several threads

A CMYK document that shows transparency is drawn as two ink planes,
and ink::compose turned them into screen colors one pixel at a time on
one thread. At 2880 x 1800 that took about as long as drawing one
plane of 50,000 paths. The renderer also searched a CMYK document for
transparency in every frame.

compose now splits the frame into bands, as many as the renderer has
threads and no more than one per 65,536 pixels. The calling thread
composes the first band and a scoped thread composes each other band,
with the same per-pixel code. The renderer keeps the answer of the
transparency test with the layers, symbol art and pattern art it was
found for, held weakly, and tests again only when the document holds
other art. With Overprint Preview on, a document whose layers or
symbols hold an overprinting fill or stroke is still tested in every
frame. FrameStats::ink_threads reports how many threads composed a
frame. On 8 threads at 2880 x 1800, a page of 300 shapes at 20% to 90%
opacity takes 30 ms a frame instead of 55 ms, and 50,000 paths with
one at 99% opacity take 258 ms instead of 361 ms, with the same
pixels.

Docs: docs/target-app-parity.md.

docs/target-app-parity.md

@@ -513,6 +513,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   cursors. DPI/hotspot and preference checks in `cursors.rs` and `tests_prefs.rs` cover #1269;
   actual OS display and Caps Lock remain unverified. Behaviour reference: [public tool-pointer documentation](https://helpx.adobe.com/illustrator/using/tools.html#change_tool_pointers).
 - **Performance:** 20k shapes + 1k texts render in 27 ms per full-retina frame (7.8 ms zoomed), 7× faster than the first version. The UI thread never blocks. The web build is 7.1 MB gzipped. Print-size images (more than 512 MB decoded, up to 2 GB) draw in the canvas and in raster exports and can be trimmed with Crop Image; an image with a side past 65,535 px is shrunk to fit, a raster export says when an image could not be decoded, and Crop Image's trim leaves CMYK images alone (#1517). On Windows `vectorcraft-cli perf` reports the cores busy and, with `--deep`, its working set, and warns on a busy machine as it does on macOS and Linux (#1502).
+  The renderer composes the two ink planes of a CMYK document that shows transparency into screen colors on up to as many threads as it rasterizes with, instead of on one. At 2880 × 1800 on 8 threads, a page of 300 shapes at 20% to 90% opacity takes 30 ms a frame instead of 55 ms, and 50,000 paths with one at 99% opacity take 258 ms instead of 361 ms (`vectorcraft-cli bench`), with the same pixels.
   The Pathfinder panel, Offset Path, Clean Up, Divide Objects Below, Make and Release Blend, Envelope Distort's Make and Release, Repeat (Radial, Grid, Mirror and Release), Vector Halftone, New and Redefine Symbol, Delete Symbol, erasing with the Symbol Sprayer, Rasterize, Convert to Artboards, Flatten Transparency, Clear Guides, Shape Builder, Live Paint, Release Image Trace, Make and Release Opacity Mask, Delete with direct-selected anchor points, Remove Anchor Points, Cut Path, and the Shaper, Knife, Eraser, Path Eraser, Line Cut, Rectangle Cut, Mirror & Cut and Blob Brush tools add and remove objects in one pass over each layer or group they change, and Outline Stroke and Expand write each new object in the place of the object it replaces (#1429).
   Delete, Cut, Group, Ungroup, Make and Release Compound Path, Make Clipping Mask, Send to Current Layer, Duplicate, Copy in the Move, Rotate, Scale, Reflect and Shear dialogs, Transform Each with Copy and each move of an Alt-drag take time in proportion to the size of the document (#1429).
   An Alt-drag with the Selection, Direct Selection, Group Selection, Rotate, Scale, Reflect or Shear tool copies the selection on its first move, and the moves after it transform those copies; the previews of the Move, Rotate, Scale, Reflect and Shear dialogs with Copy and Preview on copy the selection once in the same way (#1429).