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.toml

@@ -0,0 +1,39 @@
+# 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"]