Stabilize chord radial label boundary flips

Chord radial label rotation now follows the same normalized-angle boundary logic used by sunburst, so non-default startAngle values that place labels exactly on 90 or 270 degrees do not flip due to cosine precision around zero.

Constraint: series.startAngle is already reflected in chord node layout angles before label rotation is calculated.

Rejected: Keep using dx < 0 | floating point noise around vertical boundaries can choose the wrong flip side.

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