Keep drilled sunburst root labels visually centered

A drilled sunburst view root can become a full annular sector when rootToNode is triggered. In that state the label represents the centered root, not an angular slice, so the label should sit on the chart center without inheriting radial rotation.

Constraint: Preserve existing radial and tangential rotation for ordinary sunburst sectors and outside labels.
Rejected: Keep using the annulus midpoint | it leaves full-circle drilldown labels offset from the visual root.
Rejected: Only adjust x/y | the label can remain rotated by the previous radial orientation.
Confidence: high
Scope-risk: narrow
Directive: Do not reapply radial rotation to labels placed at the center of a full-circle sunburst sector.
Tested: git diff --check
Tested: eslint src/chart/sunburst/SunburstPiece.ts test/ut/spec/series/sunburst.test.ts
Tested: tsc --noEmit
Tested: jest --config test/ut/jest.config.cjs --coverage=false --runInBand test/ut/spec/series
Not-tested: Full visual screenshot baseline suite