Fix fill node solid colour backup update

[Annonnymmousss]

closes #3824

When the colour changes, we now compute the new fill value once and save that same new value both as the active Fill and into the correct backup slot.

Screen.Recording.2026-05-18.at.4.05.03.PM.mov

[gemini-code-assist]

Code Review

This pull request refactors the on_update closure for color inputs in the node graph properties to streamline how backup and active fill values are updated. A review comment suggests further simplifying the logic by extracting the backup index and value into a separate match expression, which would reduce code duplication and improve maintainability.

[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

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

_{Re-trigger cubic}