[v0.9] Wire up theme in Lit renderer basic catalog

[jacobsimionato]

Implement plumbing to apply the surface theme (specifically primaryColor) to components in the Lit basic catalog. This ensures consistency with the React and Angular implementations.

This is a child issue of #802.

[jacobsimionato]

Coding agent prompt: Implement plumbing to apply the surface theme (specifically primaryColor) to components in the Lit basic catalog.

Implementation Strategy

1. Discovery: Locate the Lit surface entry point in @renderers/lit/src/v0_9/.
2. Theme Application: Read the theme.primaryColor from the SurfaceModel and apply it as a CSS custom property --a2ui-primary-color to the shadow host or a top-level container.
3. Component Refactor: Update basic catalog components in @renderers/lit/src/v0_9/catalog/basic/ to consume the --a2ui-primary-color variable in their static styles.
4. Primary Use Cases:
   • Highlights for interactive elements (Buttons, Sliders, Checkboxes).
   • Active state indicators (Tabs, ChoicePickers).

Verification

• Create or update a Lit test case that initializes a surface with a custom primaryColor and asserts that the computed style of a primary button matches the theme.

Context

• @renderers/lit/src/v0_9/
• @renderers/web_core/src/v0_9/state/surface-model.ts
• @specification/v0_9/json/basic_catalog.json