Add Readability category for vector conversion diagnostics

[Copilot]

Checklist

• I have read the Contribution Guide ;
• There is an approved issue describing the change when contributing a new analyzer or suppressor ;
• I have added tests that prove my fix is effective or that my feature works ;
• I have added necessary documentation (if appropriate) ;

Short description of what this resolves:

UNT0034 and UNT0035 were miscategorized as Performance. They suggest implicit conversions between Vector2/Vector3, which offer no performance benefit—the implicit operator calls the same constructor. These are readability improvements.

Changes proposed in this pull request:

• Add DiagnosticCategory.Readability field to support readability-focused diagnostics
• Recategorize UNT0034 (Vector3→Vector2 conversion) from Performance to Readability
• Recategorize UNT0035 (Vector2→Vector3 conversion) from Performance to Readability
• Update diagnostic index documentation to reflect new categories

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