M3: group, layer and type appearance with Contents and Characters rows (M3.68)

[bflatastic]

Part of M3 (Paint & appearance). Groups, layers and type objects get an appearance of their own, as in Illustrator, with Contents and Characters rows in the Appearance panel.

• M3.68: container appearance.
  • Groups and layers carry their own fills, strokes and effects:
    • Their fills and strokes paint every member's geometry.
    • Geometry effects reshape the members as one piece.
    • Raster effects apply to the composite, so a group gets one combined shadow.
    • Knockout groups still knock their members out, and layer clipping is respected.
    • The renderer, SVG/PDF export (through baking) and Expand Appearance share one evaluator, effects::evaluate_container.
  • Type:
    • Its own fills and strokes paint under or over the characters, depending on whether they sit below or above the Characters row. This works on screen and in PDF.
    • Its object strokes are drawn on the glyph outlines.
  • Model: Appearance.contents_index: Option<usize> (None = every item above the contents). It is only written when set, and there is a native round-trip test.
  • Commands:
    • appearance.*, effect.* and graphicStyle.apply take target: "object"|"contents" and accept layer ids. They work without a selection when given ids, and error when there is no target.
    • appearance.moveItem {from: "contents"} moves the Contents slot, and contents? lets an item cross the slot in one undo step.
    • New appearance.targetContents selects a group's members.
    • appearance.addFill/addStroke on type copies the characters' paint.
  • Panel:
    • Groups and layers show a Contents row; type shows a Characters row with chips of the characters' paint.
    • Rows can be dragged across the slot.
    • Double-clicking Contents targets the members; double-clicking Characters opens the Color panel.
  • Graphic styles: a style applied to a group now sets the group's own appearance; target: "contents" styles the members as before. A group with no appearance of its own still counts as linked when all its members match.

docs/mcp.md covers targeting, the Contents slot and the commands.

Known limits:

• SVG export of live type still ignores the type object's own fills and strokes. That was already the case before this PR.
• The Layers panel has no click-to-target for layers yet; layers are reached through ids.

Checks

Rebased onto 25c8f32 (#23). cargo xtask ci passes all 7 steps on this commit stacked with the swatch libraries PR that follows it: 1,530 tests, 0 failed. It also passed on its own on e21ff60 with 1,505 tests.

New tests:

• render: one combined shadow; a group fill paints both members; a fill below Characters is hidden by the glyphs; knockout and layer fills;
• SVG bake;
• engine commands and undo;
• native round-trip;
• a headless Appearance panel test with the drop commands.

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