feat: Add comprehensive demos and tests for chain data flow

Added extensive test coverage and interactive demos for iOS chain
data flow feature to demonstrate full parity with Android WorkManager.

**New Demo: ChainDataFlowDemo** (415 lines)
- Test 1: Simple Download → Process → Upload workflow
- Test 2: HTTP GET → POST with data passing
- Test 3: Encrypt → Decrypt with IV passing
- Test 4: Parallel downloads → Single processor
- Interactive UI with logs and verification instructions
- Shows data merging in Xcode console logs

**New Tests: chain_data_flow_test.dart** (485 lines)
- 15+ unit tests covering chain builder API
- Sequential chain tests
- Parallel chain tests
- Mixed worker type chains
- Complex workflow scenarios:
  * Download-Process-Upload
  * Encrypt-Decrypt-Upload
  * Multi-download with compression
- Validates chain structure and step counts

**Demo App Integration:**
- Added new "🔗 Data Flow" tab (14th tab)
- Total tabs: 14 (was 13)
- Accessible from main demo app navigation

**Test Scenarios Covered:**
✅ FileCopy data passing (filePath, fileSize)
✅ HTTP data passing (body, statusCode, headers)
✅ Crypto data passing (encryptedPath, iv)
✅ Image processing data passing
✅ Compression data passing
✅ Parallel task data merging (last wins)
✅ Mixed worker types in single chain

**Verification:**
- iOS build: ✅ SUCCESS
- User can test interactively via demo app
- Check Xcode logs for data merging messages
- All 4 test scenarios demonstrate data flow

Author: vietnguyentuan2019