Add @example JSDoc tags to coherence scoring functions

[josedab]

What

The coherenceScore() and bpmFromHz() functions in packages/core/src/coherence.ts have JSDoc but could benefit from more complete @example blocks showing real-world usage.

Where

• packages/core/src/coherence.ts

How

1. Look at existing @example patterns in packages/core/src/hrv.ts for reference
2. Add a practical example showing how to use coherence scoring with RR intervals from a session
3. Run pnpm test && pnpm lint to verify

Acceptance Criteria

• Both functions have @example with runnable TypeScript
• pnpm test && pnpm lint pass