Add request/response logging for debugging

Implement a debug mode that logs all RPC requests and responses.

**Acceptance Criteria:**
- Add `debug: boolean` option to SoroSaveConfig
- Log request method, params, and timing when enabled
- Log response status and parsed results
- Use structured log format (JSON)
- No logging overhead when disabled

**Files:** `src/client.ts`, `src/types.ts`