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/events.py

@@ -0,0 +1,67 @@
+"""
+Event types for the mainline application.
+Defines the core events that flow through the system.
+These types support a future migration to an event-driven architecture.
+"""
+
+from dataclasses import dataclass
+from datetime import datetime
+from enum import Enum, auto
+
+
+class EventType(Enum):
+    """Core event types in the mainline application."""
+
+    NEW_HEADLINE = auto()
+    FRAME_TICK = auto()
+    MIC_LEVEL = auto()
+    NTFY_MESSAGE = auto()
+    STREAM_START = auto()
+    STREAM_END = auto()
+
+
+@dataclass
+class HeadlineEvent:
+    """Event emitted when a new headline is ready for display."""
+
+    title: str
+    source: str
+    timestamp: str
+    language: str | None = None
+
+
+@dataclass
+class FrameTickEvent:
+    """Event emitted on each render frame."""
+
+    frame_number: int
+    timestamp: datetime
+    delta_seconds: float
+
+
+@dataclass
+class MicLevelEvent:
+    """Event emitted when microphone level changes significantly."""
+
+    db_level: float
+    excess_above_threshold: float
+    timestamp: datetime
+
+
+@dataclass
+class NtfyMessageEvent:
+    """Event emitted when an ntfy message is received."""
+
+    title: str
+    body: str
+    message_id: str | None = None
+    timestamp: datetime | None = None
+
+
+@dataclass
+class StreamEvent:
+    """Event emitted when stream starts or ends."""
+
+    event_type: EventType
+    headline_count: int = 0
+    timestamp: datetime | None = None