refactor: Major base loader improvements for streaming and resilience

- Integrate state management for resume and deduplication
- Add label joining support with automatic type conversion
- Implement resilience features (retry, backpressure, rate limiting)
- Add metadata columns (_amp_batch_id) for reorg handling
- Support streaming with block ranges and reorg detection
- Separate _try_load_batch() for better error handling

Author: fordN

src/amp/loaders/__init__.py

@@ -23,7 +23,7 @@
 
 from .base import DataLoader
 from .registry import LoaderRegistry, create_loader, get_available_loaders, get_loader_class
-from .types import LoadConfig, LoadMode, LoadResult
+from .types import LabelJoinConfig, LoadConfig, LoadMode, LoadResult
 
 # Trigger auto-discovery on import
 LoaderRegistry._ensure_auto_discovery()
@@ -32,6 +32,7 @@
     'DataLoader',
     'LoadResult',
     'LoadConfig',
+    'LabelJoinConfig',
     'LoadMode',
     'LoaderRegistry',
     'get_loader_class',