feat: Add secondary reference line

[tanugarg04]

Description

Adds secondary?: boolean to the S2 component (AN-450459). Secondary lines are a lighter-emphasis variant intended for contextual values (averages, baselines) that should visually recede behind a primary target line.

Visual differences vs. primary:

• No caret caps (start and end triangles omitted)
• Rule spans full chart width (no cap-clearance offsets on either side)
• Stroke color is size-dependent: gray-600 at XS, gray-800 for S/M/L and auto mode
• Label rendered in gray-600 for all sizes

Stroke weight is unchanged — same size table as primary (XS=1px, S=1.5px, M=1.5px, L=2.5px).

LineReferenceLine.story.tsx — adds SecondaryBasic, WithSecondary, and PrimaryAndSecondary stories

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.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

🎨 Storybook -> https://opensource.adobe.com/react-spectrum-charts/PR-824

🎨 S2 Storybook -> https://opensource.adobe.com/react-spectrum-charts/PR-824-s2

📚 Docs -> https://opensource.adobe.com/react-spectrum-charts/PR-824-docs/