[M10/LNS-007] Export lens API from src/index.js

[flyingrobots]

LNS-007: Export Lens Public API

Parent: #227 (M10 LENS)

What

Export lens API from src/index.js for programmatic use:

• defineLens
• listLenses
• resetLenses
• composeLenses

Why

The public API must expose lens functionality for extensions (M12) and external consumers.

Acceptance Criteria

• All four functions exported from src/index.js
• Import test: import { defineLens, listLenses } from './src/index.js' works
• JSDoc on exports

Dependencies

• LNS-002 (registry functions)
• LNS-003 (composeLenses)

[flyingrobots]

Implemented in M10 LENS milestone. Closing as done.