Refresh chord label state rotation on updates

ChordPiece instances are reused across option updates, so a state label rotation must be refreshed every render. When a state no longer defines label.rotate, write the current normal rotation into that text state so stale state transforms do not survive later setOption calls.

Constraint: Chord labels store rotation directly on standalone Text states.

Rejected: Leave state rotation unset when missing | reused Text states can retain an older explicit rotation.

Confidence: high

Scope-risk: narrow

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

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

Tested: npm run checktype