Add bundle size optimization and tree-shaking support

Optimize the SDK bundle for minimal frontend impact.

**Acceptance Criteria:**
- Configure package.json exports map for tree-shaking
- Add sideEffects: false flag
- Split client, utils, and types into separate entry points
- Add bundle size tracking with size-limit
- Target < 50KB minified+gzipped

**Files:** `package.json`, `tsconfig.json`