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.

tests/test_viewport.py

@@ -0,0 +1,64 @@
+"""
+Tests for engine.viewport module.
+"""
+
+from engine import viewport
+
+
+class TestViewportTw:
+    """Tests for tw() function."""
+
+    def test_tw_returns_int(self):
+        """tw() returns an integer."""
+        result = viewport.tw()
+        assert isinstance(result, int)
+
+    def test_tw_positive(self):
+        """tw() returns a positive value."""
+        assert viewport.tw() > 0
+
+
+class TestViewportTh:
+    """Tests for th() function."""
+
+    def test_th_returns_int(self):
+        """th() returns an integer."""
+        result = viewport.th()
+        assert isinstance(result, int)
+
+    def test_th_positive(self):
+        """th() returns a positive value."""
+        assert viewport.th() > 0
+
+
+class TestViewportMoveTo:
+    """Tests for move_to() function."""
+
+    def test_move_to_format(self):
+        """move_to() returns correctly formatted ANSI escape."""
+        result = viewport.move_to(5, 10)
+        assert result == "\033[5;10H"
+
+    def test_move_to_default_col(self):
+        """move_to() defaults to column 1."""
+        result = viewport.move_to(5)
+        assert result == "\033[5;1H"
+
+
+class TestViewportClearScreen:
+    """Tests for clear_screen() function."""
+
+    def test_clear_screen_format(self):
+        """clear_screen() returns clear screen ANSI escape."""
+        result = viewport.clear_screen()
+        assert "\033[2J" in result
+        assert "\033[H" in result
+
+
+class TestViewportClearLine:
+    """Tests for clear_line() function."""
+
+    def test_clear_line_format(self):
+        """clear_line() returns clear line ANSI escape."""
+        result = viewport.clear_line()
+        assert result == "\033[K"