feat: add native Google Cloud Storage adapter with ADC support

Implement native @google-cloud/storage adapter for better performance
and easier authentication in Cloud Run/GCE environments.

Features:
- Application Default Credentials (ADC) for zero-config auth
- Service account authentication (keyFilename, credentials)
- HMAC fallback for backward compatibility
- Full UUID-based sharding preservation
- Write buffers for high-volume mode
- Multi-level caching and adaptive backpressure
- Complete feature parity with S3-compatible adapter

Benefits over S3-compatible GCS:
- No HMAC key management required
- Native SDK performance optimizations
- Automatic authentication in Cloud Run/GCE
- Simpler configuration

Configuration:
- type: 'gcs-native'
- gcsNativeStorage: { bucketName, keyFilename?, credentials? }
- Zero data migration required (same path structure)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -157,6 +157,7 @@
   },
   "dependencies": {
     "@aws-sdk/client-s3": "^3.540.0",
+    "@google-cloud/storage": "^7.14.0",
     "@huggingface/transformers": "^3.7.2",
     "boxen": "^8.0.1",
     "chalk": "^5.3.0",