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/widgets/packet_viewer.py

@@ -0,0 +1,56 @@
+"""
+Packet Decoder Widget - 3-panel packet inspection interface
+"""
+
+from textual.widgets import Static, DataTable, Tree
+from textual.containers import Horizontal, Vertical
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from ....analysis.core import EthernetAnalyzer
+
+
+class PacketDecoderWidget(Horizontal):
+    """
+    3-Panel Packet Decoder Interface
+    
+    Layout:
+    - Left: Flow summary tree
+    - Center: Packet list table
+    - Right: Field details tree
+    """
+    
+    def __init__(self, analyzer: 'EthernetAnalyzer', **kwargs):
+        super().__init__(**kwargs)
+        self.analyzer = analyzer
+        
+    def compose(self):
+        """Create the 3-panel layout"""
+        
+        # Left panel: Flow summary
+        with Vertical(id="flow-summary-panel"):
+            yield Static("Flow Summary", id="flow-summary-title")
+            flow_tree = Tree("Flows", id="flow-tree")
+            yield flow_tree
+        
+        # Center panel: Packet list
+        with Vertical(id="packet-list-panel"):
+            yield Static("Packet Details", id="packet-list-title")
+            packet_table = DataTable(id="packet-table")
+            packet_table.add_columns("Time", "Src", "Dst", "Protocol", "Info")
+            yield packet_table
+        
+        # Right panel: Field details
+        with Vertical(id="field-details-panel"):
+            yield Static("Field Analysis", id="field-details-title")
+            field_tree = Tree("Fields", id="field-tree")
+            yield field_tree
+    
+    def on_mount(self) -> None:
+        """Initialize the widget"""
+        self.refresh_data()
+    
+    def refresh_data(self) -> None:
+        """Refresh packet decoder data"""
+        # TODO: Implement packet data refresh
+        pass