Make chord label rotation reviewable in browser

The PR had unit coverage for radial, numeric, and state-specific chord label rotation. This adds a focused HTML regression page with project testHelper assertions so reviewers can inspect the rendered browser case.

Constraint: Reviewer feedback expects browser-level HTML cases for PR behavior changes

Confidence: high

Scope-risk: narrow

Tested: npm test -- --runInBand test/ut/spec/series/chord.test.ts

Tested: Browser loaded test/chord-label-rotate.html after dev-fast bundle and showed two checked: Pass assertions