Improve graphic List<T> fill/stroke paint handling

[YohYamasaki]

Fix/improvements on #4111.

• Fix empty color list ignoring paint target: When an empty List<Color> is passed in during svg vector rendering, the paint attribute name did not respect the paint target, and always used "fill". This is currently unreachable, but it is possible that the empty List<Color> will represent empty paint, so it is better to have this check.
• Avoid cloning renderable lists for paint presence checks: Some checks were cloning the entire graphic just to see if the paint exists, which can be avoided by borrowing.
• Make singular matrix check more robust: Add a check for when the determinant becomes +0 or -0 (not exactly 0), as this can cause overflow and the inverse matrix becomes inf.

[cubic-dev-ai]

No issues found across 3 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}

[gemini-code-assist]

Code Review

This pull request introduces a new helper function has_paint_at to check for non-blank paint attributes without cloning, simplifies fill checks, refactors stroke transparency logic, and dynamically formats paint attributes during color list rendering. The feedback suggests refactoring the newly added has_paint_at function into a single, more concise boolean expression to improve readability and remove the explicit return statement.

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