feat(examples): Add default visualization script

Add script that renders the standard Mainline visualization using the
graph-based DSL. This demonstrates the default behavior with:

- Headlines source data
- Scroll camera mode
- Terminal display
- Classic effects: noise, fade, glitch, firehose

Files added:
- examples/default_visualization.py - Main script
- examples/default_visualization.toml - TOML configuration
- examples/README.md - Documentation for all examples

Usage:
  python examples/default_visualization.py

examples/default_visualization.py

@@ -0,0 +1,86 @@
+#!/usr/bin/env python3
+"""
+Default Mainline Visualization
+
+Renders the standard Mainline visualization using the graph-based DSL.
+This demonstrates the default behavior: headlines source, scroll camera,
+terminal display, with classic effects (noise, fade, glitch, firehose).
+
+Usage:
+    python examples/default_visualization.py
+
+The visualization will be rendered once and printed to stdout.
+"""
+
+import sys
+from pathlib import Path
+
+# Add the project root to Python path
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+from engine.effects.plugins import discover_plugins
+from engine.pipeline.graph_toml import load_pipeline_from_toml
+from engine.pipeline.params import PipelineParams
+
+
+def main():
+    """Render the default Mainline visualization."""
+    print("Loading default Mainline visualization...")
+    print("=" * 70)
+
+    # Discover effect plugins
+    discover_plugins()
+
+    # Path to the TOML configuration
+    toml_path = Path(__file__).parent / "default_visualization.toml"
+
+    if not toml_path.exists():
+        print(f"Error: Configuration file not found: {toml_path}", file=sys.stderr)
+        sys.exit(1)
+
+    # Load pipeline from TOML configuration
+    try:
+        pipeline = load_pipeline_from_toml(
+            toml_path, viewport_width=80, viewport_height=24
+        )
+        print(f"✓ Pipeline loaded from {toml_path.name}")
+        print(f"  Stages: {list(pipeline._stages.keys())}")
+    except Exception as e:
+        print(f"Error loading pipeline: {e}", file=sys.stderr)
+        sys.exit(1)
+
+    # Initialize the pipeline
+    if not pipeline.initialize():
+        print("Error: Failed to initialize pipeline", file=sys.stderr)
+        sys.exit(1)
+    print("✓ Pipeline initialized")
+
+    # Set up execution context
+    ctx = pipeline.context
+    ctx.terminal_width = 80
+    ctx.terminal_height = 24
+
+    # Create params for the execution
+    params = PipelineParams(viewport_width=80, viewport_height=24)
+    ctx.params = params
+
+    # Execute the pipeline (empty items list - source will provide content)
+    print("Executing pipeline...")
+    result = pipeline.execute([])
+
+    # Render output
+    if result.success:
+        print("=" * 70)
+        print("Visualization Output:")
+        print("=" * 70)
+        for i, line in enumerate(result.data):
+            print(line)
+        print("=" * 70)
+        print(f"✓ Successfully rendered {len(result.data)} lines")
+    else:
+        print(f"Error: Pipeline execution failed: {result.error}", file=sys.stderr)
+        sys.exit(1)
+
+
+if __name__ == "__main__":
+    main()