[AN-450442] S2 Line: Direct label background rect stays opaque during highlight

[c-lamoureux]

Description

Fixes direct label background rect remaining opaque during hover/highlight dimming. Separates the opacity encoding so the background rect always uses { value: 1 } while the colored text mark uses the highlight opacity signal.

Related Issue

Jira: AN-450442 (AN-450439)

Motivation and Context

When hovering a line series, non-hovered series dim. The direct label background rect was incorrectly inheriting the highlight opacity, causing it to fade with the text instead of staying opaque to maintain label legibility.

How Has This Been Tested?

• Unit tests updated and passing (lineDirectLabelUtils.test.ts)
• Regression story added: LineDirectLabel.story.tsx (DirectLabelOpaqueBackground)
• MetricRange and Trendline confirmed unaffected
• Build and TSC verified clean

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate failed

Failed conditions
1 New Code Smells (required ≤ 0)

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[github-actions]

🎨 Storybook -> https://opensource.adobe.com/react-spectrum-charts/PR-803

📚 Docs -> https://opensource.adobe.com/react-spectrum-charts/PR-803-docs/