feat(layers): rasterize layers outside a live editor session

[hm21]

Summary

Adds LayerRasterizer and LayerRasterizerHost so layers can be captured without a running editor.

Layer.captureAsPng returns null for an unmounted layer, so layers restored from an exported state history could not be baked into a render — they deserialize fine, but they have never been laid out. The host mounts them behind its own child (painted, never visible) so capture() returns the same ExportedLayers a live session would.

final rasterizer = LayerRasterizer();

MaterialApp(
  builder: (context, child) => LayerRasterizerHost(
    rasterizer: rasterizer,
    child: child!,
  ),
);

final history = ImportStateHistory.fromMap(persistedHistory);
final captured = await rasterizer.capture(
  layers: history.stateHistory[history.editorPosition].layers,
  editorBodySize: history.lastRenderedImgSize,
);

Concurrent capture calls are serialized so they cannot read each other's repaint boundaries, and layers whose content loads asynchronously (network images, decoded assets, custom WidgetLayers) are held until awaitContentReady resolves.

Example

New page under Layers → Rasterize Layers without Editor: parses a persisted state history, rasterizes its layers with no editor involved, and shows the result both composited over the background image and layer-by-layer on a transparency grid.

Testing

• flutter test test/shared/services/layer_rasterizer/ — 6 tests, all passing
• Ran the example on macOS: 5 of 5 layers captured in ~310 ms (emoji, text, paint, and two widget layers, one of them backed by a network image that resolves through awaitContentReady)
• flutter analyze clean for package and example