Fix SplitOverflow with fullwidth characters

[bijesh-bits]

fixes #1787

• I have read the Contribution Guidelines
• I have commented on the issue above and discussed the intended changes
• A maintainer has signed off on the changes and the issue was assigned to me
• All newly added code is adequately covered by tests
• All existing tests are still running without errors
• The documentation was modified to reflect the changes OR no documentation changes are required.

Changes

• Fix Segment.SplitOverflow so it doesn’t use Substring() with cell width for fullwidth characters.
• Use cell‑width‑aware truncation for Crop and Ellipsis.
• Add unit tests covering fullwidth strings for SplitOverflow with both Ellipsis and Crop.

Testing

• dotnet test src/Spectre.Console.Tests/Spectre.Console.Tests.csproj -c Release --filter FullyQualifiedName~SegmentTests
  • Not run locally (requires .NET SDK 10.0.101 per global.json).

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