Brushes: Pattern Brush Options' corner tiles can be None or made from the side tile

Pattern Brush Options has a menu below the Outer Corner Tile and the Inner
Corner Tile. Each can be the brush's own corner art (Original), None, or one
of four tiles made from the side tile at each corner of the path:

- Auto-Centered stretches one side tile around the corner, centered on it.
- Auto-Between bends a side tile on each side into the corner, the two
  meeting on the corner's diagonal.
- Auto-Sliced lays a side tile on each side past the corner and masks it
  at the diagonal, so the two meet like a mitered frame.
- Auto-Overlap lays a side tile on each side past the corner, overlapping.

The tiles are straight at the corner and follow the path towards the side
tiles, so they meet a curve that runs into the corner. A side tile taller
than the corner can reach is stretched along until the two sides meet. On a
short path, a corner's tile is squeezed along so that it covers at most the
path up to an open path's end and half way to the next corner, and its two sides still meet on the
corner's diagonal.

The definition stores the choice as `outer_source` and `inner_source`
(`none`, `autoCentered`, `autoBetween`, `autoSliced`, `autoOverlap`), which
`brush.options` sets. Corner art wins over a source. A brush saved without
a source draws as before: a kind of corner without art takes the other
kind's art. None leaves that kind of corner empty.

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

docs/mcp.md

@@ -1502,7 +1502,11 @@ already walks Image Trace. Short agent entry points:
   `brush.new` refuses a longer one, counting the number it adds to a taken name, and `brush.options` refuses a
   longer `newName`. `brush.duplicate` does not check the length. A bristle brush that already has a longer name
   keeps it, and `brush.options` without a new name, or with a `newName` that only trims the white space around the
-  name, does not refuse. Other brush types take a name of any length.
+  name, does not refuse. Other brush types take a name of any length. A pattern brush's `outer_source` and `inner_source` set
+  what that kind of corner draws when the brush has no `outer_corner` or `inner_corner` art for it: `none` draws no
+  corner tile, and `autoCentered`, `autoBetween`, `autoSliced` and `autoOverlap` make one from the side tile. Without
+  a source, the corner draws the other kind of corner's art. `brush.options {params: {outer_corner: null}}` removes
+  the art.
 - Variables: `variable.list`, `variable.define {name, kind: text|visibility|image|graph}`, `variable.bind {variable, ids?}`,
   `variable.makeTextDynamic`, `variable.makeVisibilityDynamic`, `variable.makeImageDynamic`, `variable.makeGraphDynamic`,
   `variable.unbind`, `variable.rename`, `variable.delete`, `variable.highlight` (image values are file paths, graph

docs/target-app-parity.md

@@ -894,6 +894,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
     Document Info has a Repeat Objects category after Embedded Images. It lists each radial, grid and mirror Repeat object by name with its kind and the number of instances it makes. For a grid it gives the rows and columns instead, followed by the number of instances it draws when that is fewer. With Selection Only it lists the Repeat objects in the selection. The Document Info report and Package's report list them too (`document.info {category: "repeats"}`, `repeatObjects`).
     The Brushes panel menu's Select All Unused selects the brushes that are not used in the document, as the [Brushes](https://web.archive.org/web/20241230205225/https://helpx.adobe.com/illustrator/using/brushes.html) page (archived copy) describes. A brush counts as used when a stroke paints with it in the art (opacity masks included), in symbols, graph designs, pattern tiles or graphic styles, or in the art of a used brush (`brush.unused`).
     A rename in Brush Options and Delete Brush change every stroke that paints with the brush: in the layers and opacity masks, symbols, graph designs, pattern tiles, graphic styles and the art of other brushes. A rename gives those strokes the new name, and Delete Brush makes them plain strokes, in one undo step.
+    In Pattern Brush Options, a menu below the Outer Corner Tile and the Inner Corner Tile sets each to the brush's own corner art (Original), None, or a tile made from the side tile: Auto-Centered stretches one side tile around the corner, Auto-Between bends a side tile on each side into the corner's diagonal, Auto-Sliced cuts a side tile on each side along the diagonal, and Auto-Overlap runs a side tile on each side past the corner. Auto-Sliced masks the tiles at the diagonal, so their outlines stay as drawn. Brushes saved without these choices draw their corners as before (`brush.options` `outer_source`, `inner_source`). Evidence: `tests::pattern_auto_corners_fill_each_corner_of_a_square`, `tests::pattern_corner_source_none_leaves_that_kind_of_corner_empty` (vectorcraft-brush), `dialogs::brush_options::tests::pattern_options_set_each_corner_tile_to_original_none_or_an_automatic_tile` (vectorcraft-ui-egui).
   - **Appearance:** the Appearance panel's selected row drives every paint, stroke, gradient and transparency edit (`item` on the commands); effects apply to one fill or stroke; per-item Opacity popups, whose blend-mode list and opacity slider stay open inside them (#430; the Control bar's Stroke popover too); Mixed Appearances, Layers target dots and basic-appearance rules. Effects can be reordered, moved between items and copied by dragging; Show All Hidden Attributes; clicking an effect edits it in place; the object thumbnail drags onto art; effects render on type, images, symbols, blends, envelopes, meshes and repeats, and per-item raster effects export. Groups, layers and type have their own fills, strokes and effects (Contents and Characters rows); the Layers panel's target circles target layers, groups and objects; Expand Appearance turns fills, strokes, effects and brushes into objects.
     Fill and stroke paint (Fill, Stroke, None, Apply Last Color and Apply Last Gradient, also on characters selected with the Type tool), the Stroke panel's options, gradient and freeform point edits, Swap Fill and Stroke, Default Fill and Stroke, the Eyedropper, and the Appearance panel's item edits, Clear Appearance, Reduce to Basic Appearance, Remove Item, item reordering and Move Appearance record no undo step when they change nothing.
     An image's own fills and strokes draw the shape that a Convert to Shape or Outline Object effect on them gives them (the image's box or its frame, reshaped by the effects after it), under or over the image as its Contents row says, on the canvas, in exports and through Expand Appearance; new fills and strokes on an image go under it. Without one of those effects they draw nothing, and Use Preview Bounds measures an image by its frame.