Phase 6: Production hardening with logging, monitoring, and enhanced documentation

## Changes

### New Files
- **PaykitLogger.kt**: Structured logging utility with:
  - Configurable log levels (DEBUG, INFO, WARNING, ERROR, NONE)
  - Performance metrics logging
  - Payment flow event tracking
  - Privacy-safe logging (payment details only in DEBUG)
  - Integration with PaykitConfigManager error reporting

### Enhanced Documentation
- **README.md**: Added Phase 6 section with:
  - Logging & monitoring guide
  - Error reporting integration examples (Firebase Crashlytics, Sentry)
  - Retry logic configuration
  - Performance optimization details
  - Security features overview (including EncryptedSharedPreferences)
  - Production deployment guide with rollout strategy
  - ProGuard rules reference
  - Known limitations documentation

## Features

### Logging
- Structured logging with categories and context
- Android Log integration with proper tag hierarchy
- Automatic error reporting to configured monitoring service
- Privacy controls for payment data

### Monitoring
- Performance metrics for all operations (timing in milliseconds)
- Payment flow event tracking
- Error context with stack traces
- Integration-ready for Firebase Crashlytics, Sentry, etc.

### Configuration
- Environment-based configuration (DEVELOPMENT/STAGING/PRODUCTION)
- Configurable timeouts and retry behavior
- Error reporter callback for monitoring integration
- Log level filtering with Kotlin enum comparison

## Testing
All existing tests pass. No breaking changes to APIs.

## Production Ready
- Error handling with context
- Retry logic with exponential backoff
- Privacy-safe logging
- Security best practices documented (encrypted storage, ProGuard)
- Gradual rollout strategy defined

Ref: Phase 6 of Paykit Production Integration Plan

Author: JOHN