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

completion/mainline-completion.bash

@@ -70,6 +70,12 @@ _mainline_completion() {
             COMPREPLY=($(compgen -W "demo demo-base demo-pygame demo-camera-showcase poetry headlines empty test-basic test-border test-scroll-camera" -- "${cur}"))
             return
             ;;
+
+        --positioning)
+            # Positioning modes
+            COMPREPLY=($(compgen -W "absolute relative mixed" -- "${cur}"))
+            return
+            ;;
     esac
 
     # Flag completion (start with --)
@@ -85,6 +91,7 @@ _mainline_completion() {
             --viewport
             --preset
             --theme
+            --positioning
             --websocket
             --websocket-port
             --allow-unsafe