refactor: phase 2 - modularization of scroll engine

Split monolithic scroll.py into focused modules:
- viewport.py: terminal size (tw/th), ANSI positioning helpers
- frame.py: FrameTimer class, scroll step calculation
- layers.py: message overlay, ticker zone, firehose rendering
- scroll.py: simplified orchestrator, imports from new modules

Add stream controller and event types for future event-driven architecture:
- controller.py: StreamController for source initialization and stream lifecycle
- events.py: EventType enum and event dataclasses (HeadlineEvent, FrameTickEvent, etc.)

Added tests for new modules:
- test_viewport.py: 8 tests for viewport utilities
- test_frame.py: 10 tests for frame timing
- test_layers.py: 13 tests for layer compositing
- test_events.py: 11 tests for event types
- test_controller.py: 6 tests for stream controller

This enables:
- Testable chunks with clear responsibilities
- Reusable viewport utilities across modules
- Better separation of concerns in render pipeline
- Foundation for future event-driven architecture

Also includes Phase 1 documentation updates in code comments.

engine/controller.py

@@ -0,0 +1,46 @@
+"""
+Stream controller - manages input sources and orchestrates the render stream.
+"""
+
+from engine.config import Config, get_config
+from engine.mic import MicMonitor
+from engine.ntfy import NtfyPoller
+from engine.scroll import stream
+
+
+class StreamController:
+    """Controls the stream lifecycle - initializes sources and runs the stream."""
+
+    def __init__(self, config: Config | None = None):
+        self.config = config or get_config()
+        self.mic: MicMonitor | None = None
+        self.ntfy: NtfyPoller | None = None
+
+    def initialize_sources(self) -> tuple[bool, bool]:
+        """Initialize microphone and ntfy sources.
+
+        Returns:
+            (mic_ok, ntfy_ok) - success status for each source
+        """
+        self.mic = MicMonitor(threshold_db=self.config.mic_threshold_db)
+        mic_ok = self.mic.start() if self.mic.available else False
+
+        self.ntfy = NtfyPoller(
+            self.config.ntfy_topic,
+            reconnect_delay=self.config.ntfy_reconnect_delay,
+            display_secs=self.config.message_display_secs,
+        )
+        ntfy_ok = self.ntfy.start()
+
+        return bool(mic_ok), ntfy_ok
+
+    def run(self, items: list) -> None:
+        """Run the stream with initialized sources."""
+        if self.mic is None or self.ntfy is None:
+            self.initialize_sources()
+        stream(items, self.ntfy, self.mic)
+
+    def cleanup(self) -> None:
+        """Clean up resources."""
+        if self.mic:
+            self.mic.stop()