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

presets.toml

@@ -53,6 +53,18 @@ viewport_height = 24
 # DEMO PRESETS (for demonstration and exploration)
 # ============================================
 
+[presets.upstream-default]
+description = "Upstream default operation (terminal display, legacy behavior)"
+source = "headlines"
+display = "terminal"
+camera = "scroll"
+effects = ["noise", "fade", "glitch", "firehose"]
+camera_speed = 1.0
+viewport_width = 80
+viewport_height = 24
+enable_message_overlay = false
+positioning = "mixed"
+
 [presets.demo-base]
 description = "Demo: Base preset for effect hot-swapping"
 source = "headlines"
@@ -63,17 +75,19 @@ camera_speed = 0.1
 viewport_width = 80
 viewport_height = 24
 enable_message_overlay = true
+positioning = "mixed"
 
 [presets.demo-pygame]
 description = "Demo: Pygame display version"
 source = "headlines"
 display = "pygame"
 camera = "feed"
-effects = []  # Demo script will add/remove effects dynamically
+effects = ["noise", "fade", "glitch", "firehose"]  # Default effects
 camera_speed = 0.1
 viewport_width = 80
 viewport_height = 24
 enable_message_overlay = true
+positioning = "mixed"
 
 [presets.demo-camera-showcase]
 description = "Demo: Camera mode showcase"
@@ -85,6 +99,7 @@ camera_speed = 0.5
 viewport_width = 80
 viewport_height = 24
 enable_message_overlay = true
+positioning = "mixed"
 
 [presets.test-message-overlay]
 description = "Test: Message overlay with ntfy integration"
@@ -96,6 +111,7 @@ camera_speed = 0.1
 viewport_width = 80
 viewport_height = 24
 enable_message_overlay = true
+positioning = "mixed"
 
 # ============================================
 # SENSOR CONFIGURATION