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.

crates/engine/src/cmd/brushsym.rs

@@ -134,7 +134,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Brush Options…",
             [],
             None,
-            "{name, params?: {…fields to change (brush.get shows them; values outside their ranges are clamped). calligraphic: angle −180..180°, roundness 0..100%, size 0..1296 pt, variation: [angle°, roundness%, size pt], modes: [fixed|random|pressure for angle, roundness, size] (pressure goes from value − variation at the lightest pen pressure to value + variation at the heaviest). scatter: size [a, b] 1..10000%, spacing [a, b] 1..10000%, scatter [a, b] −1000..1000%, rotation [a, b] −180..180°, modes: [fixed|random|pressure for size, spacing, scatter, rotation] (fixed: a; random: between a and b; pressure: a at the lightest pen pressure to b at the heaviest), rotation_relative_to_path. art: width 1..1000%, width_mode: null (fixed)|pressure, width_max 1..1000% (pressure: width at the lightest pen pressure to width_max at the heaviest, half way on a stroke drawn without pressure), scale: {mode: proportional|stretch|betweenGuides, start, end: 0..1 of the art length}, direction: leftToRight|rightToLeft|topToBottom|bottomToTop, flip_along, flip_across, overlap. pattern: scale 1..10000%, spacing 0..10000%, fit: stretch|addSpace|approximate, flip_along, flip_across. scatter, art and pattern: colorization: {method: none|tints|tintsAndShades|hueShift, key: colour (hueShift: the art colour that becomes the stroke colour; without a key: the solid color the art's fills and strokes use most, black when they use none)}; colorization and the art brush's scale change only the fields given}, newName? (trimmed; blank: the name is kept; a new bristle brush name has at most 31 characters, and a longer name the brush has is kept)} edit a brush definition (strokes using it update) → {name}",
+            "{name, params?: {…fields to change (brush.get shows them; values outside their ranges are clamped). calligraphic: angle −180..180°, roundness 0..100%, size 0..1296 pt, variation: [angle°, roundness%, size pt], modes: [fixed|random|pressure for angle, roundness, size] (pressure goes from value − variation at the lightest pen pressure to value + variation at the heaviest). scatter: size [a, b] 1..10000%, spacing [a, b] 1..10000%, scatter [a, b] −1000..1000%, rotation [a, b] −180..180°, modes: [fixed|random|pressure for size, spacing, scatter, rotation] (fixed: a; random: between a and b; pressure: a at the lightest pen pressure to b at the heaviest), rotation_relative_to_path. art: width 1..1000%, width_mode: null (fixed)|pressure, width_max 1..1000% (pressure: width at the lightest pen pressure to width_max at the heaviest, half way on a stroke drawn without pressure), scale: {mode: proportional|stretch|betweenGuides, start, end: 0..1 of the art length}, direction: leftToRight|rightToLeft|topToBottom|bottomToTop, flip_along, flip_across, overlap. pattern: scale 1..10000%, spacing 0..10000%, fit: stretch|addSpace|approximate, flip_along, flip_across, outer_corner and inner_corner: corner tile art or null, outer_source and inner_source: what that kind of corner draws without its art: none|autoCentered|autoBetween|autoSliced|autoOverlap (a tile made from the side tile), or null (the other kind of corner's art). scatter, art and pattern: colorization: {method: none|tints|tintsAndShades|hueShift, key: colour (hueShift: the art colour that becomes the stroke colour; without a key: the solid color the art's fills and strokes use most, black when they use none)}; colorization and the art brush's scale change only the fields given}, newName? (trimmed; blank: the name is kept; a new bristle brush name has at most 31 characters, and a longer name the brush has is kept)} edit a brush definition (strokes using it update) → {name}",
             has_doc,
             brush_options
         ),