Keep ordinary sunburst annular labels in their own ring

The review correctly pointed out that the previous r0 guard had been broadened too far. The drilldown view root still needs centered full-circle label placement, but ordinary annular full-circle descendants should keep their midpoint label position.

Constraint: Review noted the removed layout.r0 === 0 guard changed non-view-root annular full-circle sectors
Rejected: Center every full-circle sector | single-child annular descendants would collapse labels to the chart center
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Only root disks and the current sunburst view root should use chart-center full-circle labels
Tested: ./node_modules/.bin/jest --config test/ut/jest.config.cjs test/ut/spec/series/sunburst.test.ts --runInBand --coverage=false; npm run checktype; ./node_modules/.bin/eslint src/chart/sunburst/SunburstPiece.ts test/ut/spec/series/sunburst.test.ts; git diff --check
Not-tested: full visual browser pass