Enhanced Textual TUI with proper API usage and documentation

- Fixed DataTable row selection and event handling
- Added explicit column keys to prevent auto-generated keys
- Implemented row-to-flow mapping for reliable selection tracking
- Converted left metrics panel to horizontal top bar
- Fixed all missing FlowStats/EnhancedAnalysisData attributes
- Created comprehensive Textual API documentation in Documentation/textual/
- Added validation checklist to prevent future API mismatches
- Preserved cursor position during data refreshes
- Fixed RowKey type handling and event names

The TUI now properly handles flow selection, displays metrics in a compact top bar,
and correctly correlates selected rows with the details pane.

analyzer/tui/textual/__init__.py

@@ -0,0 +1,8 @@
+"""
+Textual-based TUI implementation for StreamLens
+Provides modern, reactive interface with widget-based architecture
+"""
+
+from .app import StreamLensApp
+
+__all__ = ['StreamLensApp']