fix(mix): preserve BoxShadow blur style

[leoafarias]

Related issue

Closes #990

Description

Preserves non-default BoxShadow.blurStyle values through the complete BoxShadowMix lifecycle so converting and resolving a Flutter box shadow is lossless.

Changes

• Add public constructor, factory, fluent setter, conversion, merge, resolution, diagnostics, and equality support for blurStyle.
• Add regression coverage for non-default round-tripping, maybeValue, merge precedence, and equality.
• Classify the new field as outside the existing Mix Protocol v1 wire contract without changing that format.

Review Checklist

• Testing: Focused tests, workspace generation, full CI, schema inventory, Dart analysis, and DCM analysis pass.
• Breaking Changes: No breaking changes; the API addition is additive.
• Documentation Updates: Updated the public API documentation; no separate guide changes are required.
• Website Updates: No website updates are required.

Additional Information (optional)

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