feat(bar): add per-bar colors support

[hdcodedev]

Why

Fixes #139

Bar charts currently support only a single bar color, which limits visual differentiation for multi-series-like categorical data and demos.

Summary

This change adds optional per-bar color support to BarChartStyle and rendering, while preserving existing behavior when custom colors are not provided. It validates that the provided color list matches the source data size and surfaces a chart error when mismatched. In compact dense mode, colors are remapped to the bucket center bars so rendered bars keep deterministic color selection. Demo fixtures, screenshot coverage, and UI/unit tests were updated for the new behavior.

Changes

• Add barColors: List<Color> to BarChartStyle and BarChartDefaults.style(...)
• Validate barColors count against data points and show mismatch error
• Render per-bar colors with fallback to default barColor
• Map source colors to compact-density buckets using center indices
• Add tests and screenshot references for custom per-bar colors

Release/Changeset

• Created: ../charts-docs/content/snapshot/changes/406-bar-chart-per-bar-colors.md
• Docs PR: docs(changeset): add changeset for charts pr #406 charts-docs#13
• Playground PR: feat(bar): support per-bar color palettes in playground charts-playground#11

Notes/Risk

• None

[github-actions]

✅ Test Summary

Modules

• ✅ Charts: 227 tests completed
• ✅ Android screenshot: 104 tests completed

CI Behavior

• ⚪ CI behavior (total): 0 tests completed

Overall

• ✅ Total: 331 tests completed successfully