Make drilled sunburst root centering reviewable

The PR had unit coverage for drilldown root label centering. This adds a browser-level HTML regression case that drills into a full annular root sector and asserts the label remains centered with zero auto rotation.

Constraint: Reviewer feedback expects browser-level HTML cases for visual layout fixes

Confidence: high

Scope-risk: narrow

Tested: npm test -- --runInBand test/ut/spec/series/sunburst.test.ts

Tested: Browser loaded test/sunburst-drilldown-root-label.html after dev-fast bundle and showed checked: Pass