feat(presets): Add upstream-default preset and enhance demo preset

- Add upstream-default preset matching upstream mainline behavior:
  - Terminal display (not pygame)
  - No message overlay
  - Classic effects: noise, fade, glitch, firehose
  - Mixed positioning mode

- Enhance demo preset to showcase sideline features:
  - Hotswappable effects via effect plugins
  - LFO sensor modulation (oscillator sensor)
  - Mixed positioning mode
  - Message overlay with ntfy integration
  - Includes hud effect for visual feedback

- Update all presets to use mixed positioning mode
- Update completion script for --positioning flag

Usage:
  python -m mainline --preset upstream-default --display terminal
  python -m mainline --preset demo --display pygame

engine/app/pipeline_runner.py

@@ -139,6 +139,16 @@ def run_pipeline_mode(preset_name: str = "demo"):
                 print("Error: Invalid viewport format. Use WxH (e.g., 40x15)")
                 sys.exit(1)
 
+    # Set positioning mode from command line or config
+    if "--positioning" in sys.argv:
+        idx = sys.argv.index("--positioning")
+        if idx + 1 < len(sys.argv):
+            params.positioning = sys.argv[idx + 1]
+    else:
+        from engine import config as app_config
+
+        params.positioning = app_config.get_config().positioning
+
     pipeline = Pipeline(config=preset.to_config())
 
     print("  \033[38;5;245mFetching content...\033[0m")