feat: S2 Line hover value label

[tanugarg04]

Research spike - Implements the on-hover metric value label for S2 Line as a research spike. The label appears adjacent to the hovered data point by default, without requiring any child component. Includes line thickening on hover, a configurable label key, dimension hover support, and a shared text-mark utility that keeps hover labels and LinePointAnnotation visually in sync.

Description

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.