feat: Add gallery presets, MultiDisplay support, and viewport tests

- Add ~20 gallery presets covering sources, effects, cameras, displays
- Add MultiDisplay support with --display multi:terminal,pygame syntax
- Fix ViewportFilterStage to recompute layout on viewport_width change
- Add benchmark.py module for hook-based performance testing
- Add viewport resize tests to test_viewport_filter_performance.py

tests/test_app.py

@@ -30,11 +30,11 @@ class TestMain:
             patch("engine.app.run_pipeline_mode") as mock_run,
         ):
             mock_config.PIPELINE_DIAGRAM = False
-            mock_config.PRESET = "border-test"
+            mock_config.PRESET = "gallery-sources"
             mock_config.PIPELINE_MODE = False
             sys.argv = ["mainline.py"]
             main()
-            mock_run.assert_called_once_with("border-test")
+            mock_run.assert_called_once_with("gallery-sources")
 
     def test_main_exits_on_unknown_preset(self):
         """main() exits with error for unknown preset."""
@@ -120,11 +120,11 @@ class TestRunPipelineMode:
             mock_create.return_value = mock_display
 
             try:
-                run_pipeline_mode("border-test")
+                run_pipeline_mode("gallery-display-terminal")
             except (KeyboardInterrupt, SystemExit):
                 pass
 
-            # Verify display was created with 'terminal' (preset display for border-test)
+            # Verify display was created with 'terminal' (preset display)
             mock_create.assert_called_once_with("terminal")
 
     def test_run_pipeline_mode_respects_display_cli_flag(self):