[Feature Request]: Pattern for better accessibility in PieChart

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# Description

To improve accessibility of PieChart it would help to have an optional pattern e.g. dots to make pie chart sections or segments easier to distinguish for people who have trouble with low contrast.

# Example

# Proposed Solution

```dart
class PieChartSectionData {
// ...
final DotPattern pattern;
// ...
}

class PieChartStackSegmentData {
// ...
final DotPattern pattern;
// ...
}

class DotPattern {
final Color color;
final double spacing;
final double dotRadius;
}
```

Start with the PieChartSectionData and PieChartStackSegmentData entry points described in the issue, then trace how PieChart sections are rendered. Review the proposed DotPattern fields and define done as optional patterns that make segments easier to distinguish while preserving existing chart behavior.

Written by the indexing model from the issue text.