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/README.md

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+# Examples
+
+This directory contains example scripts demonstrating how to use Mainline's features.
+
+## Default Visualization
+
+**`default_visualization.py`** - Renders the standard Mainline visualization using the graph-based DSL.
+
+```bash
+python examples/default_visualization.py
+```
+
+This script demonstrates:
+- Graph-based pipeline configuration using TOML
+- Default Mainline behavior: headlines source, scroll camera, terminal display
+- Classic effects: noise, fade, glitch, firehose
+- One-shot rendering (prints to stdout)
+
+### Configuration
+
+The visualization is defined in `default_visualization.toml`:
+
+```toml
+[nodes.source]
+type = "source"
+source = "headlines"
+
+[nodes.camera]
+type = "camera"
+mode = "scroll"
+speed = 1.0
+
+[nodes.noise]
+type = "effect"
+effect = "noise"
+intensity = 0.3
+
+[nodes.fade]
+type = "effect"
+effect = "fade"
+intensity = 0.5
+
+[nodes.display]
+type = "display"
+backend = "terminal"
+
+[connections]
+list = ["source -> camera -> noise -> fade -> display"]
+```
+
+## Graph DSL Demonstration
+
+**`graph_dsl_demo.py`** - Demonstrates the graph-based DSL in multiple ways:
+
+```bash
+python examples/graph_dsl_demo.py
+```
+
+Shows:
+- Imperative Python API for building graphs
+- Dictionary-based API
+- Graph validation (cycles, disconnected nodes)
+- Different node types and configurations
+
+## Integration Test
+
+**`test_graph_integration.py`** - Tests the graph system with actual pipeline execution:
+
+```bash
+python examples/test_graph_integration.py
+```
+
+Verifies:
+- Graph loading from TOML
+- Pipeline execution
+- Output rendering
+- Comparison with preset-based pipelines
+
+## Other Demos
+
+- **`demo-lfo-effects.py`** - LFO modulation of effect intensities (Pygame display)
+- **`demo_oscilloscope.py`** - Oscilloscope visualization
+- **`demo_image_oscilloscope.py`** - Image-based oscilloscope
+
+## Graph DSL Reference
+
+See `docs/graph-dsl.md` for complete documentation on the graph-based DSL syntax.

examples/default_visualization.py

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+#!/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()

examples/default_visualization.toml

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+# Default Mainline Visualization
+# This configuration renders the standard Mainline visualization using the
+# graph-based DSL. It matches the upstream-default preset behavior.
+
+[nodes.source]
+type = "source"
+source = "headlines"
+
+[nodes.camera]
+type = "camera"
+mode = "scroll"
+speed = 1.0
+
+[nodes.noise]
+type = "effect"
+effect = "noise"
+intensity = 0.3
+
+[nodes.fade]
+type = "effect"
+effect = "fade"
+intensity = 0.5
+
+[nodes.glitch]
+type = "effect"
+effect = "glitch"
+intensity = 0.2
+
+[nodes.firehose]
+type = "effect"
+effect = "firehose"
+intensity = 0.4
+
+[nodes.display]
+type = "display"
+backend = "terminal"
+
+[connections]
+list = ["source -> camera -> noise -> fade -> glitch -> firehose -> display"]