fix(app): --display CLI flag now takes priority over preset

The --display CLI flag wasn't being checked, so it was always using
the preset's display backend.

Now app.py checks if --display was provided and uses it if present,
otherwise falls back to the preset's display setting.

Example:
  uv run mainline.py --preset border-test --display websocket
  # Now correctly uses websocket instead of terminal (border-test default)

engine/app.py

@@ -110,9 +110,17 @@ def run_pipeline_mode(preset_name: str = "demo"):
 
         print(f"  \033[38;5;82mLoaded {len(items)} items\033[0m")
 
-    display = DisplayRegistry.create(preset.display)
+    # CLI --display flag takes priority over preset
+    # Check if --display was explicitly provided
+    display_name = preset.display
+    if "--display" in sys.argv:
+        idx = sys.argv.index("--display")
+        if idx + 1 < len(sys.argv):
+            display_name = sys.argv[idx + 1]
+
+    display = DisplayRegistry.create(display_name)
     if not display:
-        print(f"  \033[38;5;196mFailed to create display: {preset.display}\033[0m")
+        print(f"  \033[38;5;196mFailed to create display: {display_name}\033[0m")
         sys.exit(1)
 
     display.init(80, 24)
@@ -166,7 +174,7 @@ def run_pipeline_mode(preset_name: str = "demo"):
                 f"effect_{effect_name}", create_stage_from_effect(effect, effect_name)
             )
 
-    pipeline.add_stage("display", create_stage_from_display(display, preset.display))
+    pipeline.add_stage("display", create_stage_from_display(display, display_name))
 
     pipeline.build()