PDF/SVG export: knockout groups nest no deeper with more elements

A knockout group was written as a chain of soft masks, one inside the
next for each element, so a group of about 200 elements made a PDF that
VectorCraft's own PDF import refused (form nesting past 50) and an SVG
that the web build refused as too deep.

- PDF writes a knockout group as one transparency group with /K true,
  with /I true when the group isolates, and PDF import reads it back as
  a knockout group. An element with several paints is drawn as one
  group inside it, so its own fills and strokes do not knock each other
  out.
- Where an element knocks out less than the area it paints (its opacity
  or mask defines its knockout shape, or it paints translucently), each
  element is drawn through one soft mask of the overlapping elements
  above it. Past 65,536 objects in a group's masks, or 2,048 elements,
  the group is a PDF knockout group, with a warning.
- SVG draws each element through one mask of the overlapping elements
  above it, built from one shared copy of each element's shape, so the
  elements stay siblings. Past 65,536 shapes in a group's masks, or
  2,048 elements, the group is written as a plain group, with a warning.

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

docs/mcp.md

@@ -2428,8 +2428,12 @@ neutral, the default). In a knockout group each child hides what it covers of th
 pass the enclosing group's setting to their children, off groups never knock out. `knockoutShape: true` makes the
 object's opacity and opacity mask scale how much it knocks out. `transparency.togglePageKnockoutGroup` and
 `transparency.togglePageIsolatedBlending` (`{value?}`) treat the whole page as a knockout or isolated group; they are
-saved with the document and undoable, and `transparency.info` reports them. PDF and SVG write knockout groups as
-soft-masked groups with the same look (the PDF export reports a warning).
+saved with the document and undoable, and `transparency.info` reports them. PDF writes a knockout group as a PDF
+knockout group, which PDF import reads back as one. Where an element knocks out less than the area it paints
+(`knockoutShape: true` with opacity or a mask, or translucent paint), PDF draws each element through one soft mask of
+the overlapping elements above it, as SVG does for every knockout group of translucent art. A group whose masks would
+hold more than 65,536 objects (PDF) or shapes (SVG), or that has more than 2,048 elements, is written as a PDF knockout
+group or a plain SVG group instead, and the export reports a warning.
 
 Groups are not isolated unless Isolate Blending is on: a blend mode inside a group with opacity, a blend mode, an
 opacity mask, knockout or a clip reaches the art below the group, on screen and in PDF (a knockout group's elements

docs/target-app-parity.md

@@ -1511,6 +1511,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - **Transparency:** three-state Knockout Group (on, neutral, off), Opacity & Mask Define Knockout Shape, Page Isolated Blending and Page Knockout Group, on screen and in SVG/PDF. SVG export writes a knockout group of opaque art plainly, without masks, so shapes meeting edge to edge leave no seam; translucent, blended, masked or brushed art knocks out through masks. Blend modes match the standard formulas on screen, in SVG and in PDF, and non-isolated groups blend with the art below. Flatten Transparency with presets, a dialog and a Flattener Preview panel. Flattening for files without transparency (PDF/X-1a, PDF/X-3) in a CMYK document keeps the inks where it rasterizes, as under an image with see-through pixels: the images it makes hold ink amounts, not screen colours the file would separate again, so a C100 M6 Y41 K3 panel stays C100 M6 Y41 K3 there (#2160, `engine/src/cmd/fileio/tests_pdfx.rs`).
     On the canvas, the renderer no longer draws the picture of the art below back as an image each time it copies the backdrop of a non-isolated group that blends: it keeps that picture and draws the art that follows over it. At 2880 × 1800, 50 translucent groups with Multiply members take 65 ms a frame instead of 299 ms on 8 threads (`vectorcraft-cli bench`), and the frames have the same pixels.
     Type inside a group, layer, symbol instance, blend or repeat keeps its opacity, blend mode, opacity mask, Opacity & Mask Define Knockout Shape, own effects, perspective and visibility when the effects on the container outline it. Hidden type with a geometry effect of its own is left out of exports.
+    PDF export writes a knockout group as one PDF knockout group (`/K true`, with `/I true` when the group isolates; ISO 32000-1, 11.4.6.3), so a knockout group of any size opens again in VectorCraft as a knockout group, and the form nesting of the file does not grow with the number of elements. When an element's opacity or mask defines its knockout shape, or it paints translucently, each element is drawn through one soft mask of the overlapping elements above it instead; past 65,536 objects in a group's masks (or 2,048 elements) the group is a PDF knockout group, with a warning that its translucent elements knock out fully. SVG export draws each element of a knockout group through one mask of the overlapping elements above it, and the masks refer to one copy of each element's shape, so the elements stay siblings and the file nests no deeper with more elements; past 65,536 shapes in a group's masks (or 2,048 elements) the group is written as a plain group, with a warning. Evidence: `pdf/src/tests/blend.rs`, `svg/tests/it/blend_export.rs`, `pdf/tests/gradient_forms.rs` (a gradient in a knockout group reads back in place).
   - **Graphic styles:** styles keep opacity and blend mode, capture groups and type, apply on top with Alt, and link to the objects using them; Redefine, Break Link, Graphic Style Options, Select All Unused, Sort by Name, and Select > Same Graphic Style / Appearance Attribute. The panel has list and thumbnail views with rendered previews, merge, reorder and drag and drop, plus a Control bar Style picker; ten generated style libraries (including Game HUD, Status Effects, Painted and Sketched, whose seven styles each repaint a solid fill with one watercolor, dry brush, pastel, charcoal or ink effect, and Textures) save and load as `.vcstyles`.
     The built-in Textures graphic style library (`graphicStyle.addFromLibrary {library: "textures"}`) has eight styles that paint their fills and strokes with the texture effects: Fine Grain (Grain), Canvas Weave and Burlap Sack (Texturizer), Cracked Glaze (Craquelure), Mosaic Floor (Mosaic Tiles), Quilted Squares (Patchwork), Leaded Glass (Stained Glass) and Damp Paper (Water Paper). The help page names new graphic styles for textures ([What's new](https://helpx.adobe.com/illustrator/desktop/new-features/whats-new.html)) without naming them, so the styles, their names and their settings are VectorCraft's own (`tests::stylelib::textures_styles_apply_their_texture_effects_and_keep_their_params_through_save_and_open`).
     In the Graphic Styles panel, Shift-click selects a range of styles and Ctrl/Cmd-click adds or removes one.