feat: Add Scatter to the S2 package (pre-alpha)

[tanugarg04]

Add Scatter to the S2 package (pre-alpha)

Description

Ports Scatter into @spectrum-charts/react-spectrum-charts-s2 as a pre-alpha
component — full prop support, plus its child components ScatterPath,
ScatterAnnotation, Trendline, and TrendlineAnnotation. Also renames the
package's rc export tier to pre-alpha (./rc → ./pre-alpha) and fixes two
pre-existing bugs uncovered along the way.

The underlying vega-spec-builder-s2 spec-generation logic for all of these
already existed and needed no changes — this PR is entirely the React
component/adapter layer plus build/docs wiring.

Related Issue

Motivation and Context

How Has This Been Tested?

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.

[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-869

🎨 S2 Storybook -> https://opensource.adobe.com/react-spectrum-charts/PR-869-s2

📚 Docs -> https://opensource.adobe.com/react-spectrum-charts/PR-869-docs/