Migrate VR scenarios to automatic theme variants

Is your feature request related to a problem? Please describe. Standalone visual-regression scenarios currently mostly exercise only legacy defaults, leaving their new theme behavior unprotected.

Describe the solution you'd like Migrate the ordinary non-www, non-theme specs to the automatic legacy/light/dark runner from #7736.

For each migrated assertion, keep the legacy baseline, add independently reviewed light and dark baselines, and rely on automatic variant snapshot naming. Do not encode the selected variant in the test title. Use the supported structured exception mechanism only for a scenario whose purpose makes an outer automatic theme invalid.

Keep the work in reviewable chart-family PRs; do not combine it with the dedicated custom-theme suite or website examples.

Describe alternatives you've considered Leaving these scenarios to the dedicated theme stories alone would miss real component combinations and chart-specific rendering paths.

A bulk baseline update across all chart types would make unintended differences difficult to identify.

Additional context Blocked by #7736. Dedicated tests under test-vr/tests/theme/** are intentionally excluded because their explicit providers require an audit before automatic wrapping.

Acceptance criteria:

• Every ordinary in-scope specialized-chart screenshot has legacy, light, and dark baselines.
• Baselines remain separate per browser and per variant.
• Migration PRs preserve existing legacy expectations and clearly review the added themed expectations.
• No story-specific testTheme plumbing is introduced.

For convenience, use the agentic skill https://github.com/recharts/recharts/blob/main/.agents/skills/vr-test-migration/SKILL.md