Make dense chord labels readable around the ring

Chord labels accepted the shared label.rotate shape but rendered every node label horizontally. This wires radial rotation into the chord label placement path and keeps numeric rotation working through the same branch, with a regression test that inspects the rendered node text rotation.

Constraint: Issue #21199 asks specifically for label.rotate: 'radial' on chord labels.

Rejected: Reuse pie label layout wholesale | chord labels do not use pie guide-line collision layout and only need the sector-midline rotation rule.

Confidence: high

Scope-risk: narrow

Directive: Keep chord radial rotation aligned with pie's upright text flip unless chord label geometry changes.

Tested: npx jest --config test/ut/jest.config.cjs --runTestsByPath test/ut/spec/series/chord.test.ts --coverage=false

Tested: npm test -- --runInBand --coverage=false

Tested: npm run checktype

Tested: npm run lint -- --no-cache src/chart/chord/ChordPiece.ts src/chart/chord/ChordSeries.ts

Tested: npx eslint --no-cache test/ut/spec/series/chord.test.ts

Tested: npx tsc -p test/ut/tsconfig.json --noEmit --moduleResolution node --skipLibCheck

Not-tested: npm run checkheader currently fails on pre-existing header gaps in test/ut/coverage/*, test/line-area-empty-dimension-name.html, and test/visualmap-seriesTargets.html.