feat(preset): Add enable_message_overlay to presets

- Add enable_message_overlay field to PipelinePreset dataclass
- Enable message overlay in demo, ui, and firehose presets
- Add test-message-overlay preset for testing
- Update preset loader to support new field from TOML files

engine/pipeline/presets.py

@@ -59,6 +59,7 @@ class PipelinePreset:
     viewport_height: int = 24  # Viewport height in rows
     source_items: list[dict[str, Any]] | None = None  # For ListDataSource
     enable_metrics: bool = True  # Enable performance metrics collection
+    enable_message_overlay: bool = False  # Enable ntfy message overlay
 
     def to_params(self) -> PipelineParams:
         """Convert to PipelineParams (runtime configuration)."""
@@ -113,6 +114,7 @@ class PipelinePreset:
             viewport_height=data.get("viewport_height", 24),
             source_items=data.get("source_items"),
             enable_metrics=data.get("enable_metrics", True),
+            enable_message_overlay=data.get("enable_message_overlay", False),
         )
 
 
@@ -124,6 +126,7 @@ DEMO_PRESET = PipelinePreset(
     display="pygame",
     camera="scroll",
     effects=["noise", "fade", "glitch", "firehose"],
+    enable_message_overlay=True,
 )
 
 UI_PRESET = PipelinePreset(
@@ -134,6 +137,7 @@ UI_PRESET = PipelinePreset(
     camera="scroll",
     effects=["noise", "fade", "glitch"],
     border=BorderMode.UI,
+    enable_message_overlay=True,
 )
 
 POETRY_PRESET = PipelinePreset(
@@ -170,6 +174,7 @@ FIREHOSE_PRESET = PipelinePreset(
     display="pygame",
     camera="scroll",
     effects=["noise", "fade", "glitch", "firehose"],
+    enable_message_overlay=True,
 )
 
 FIXTURE_PRESET = PipelinePreset(