feat(core): add Camera abstraction for viewport scrolling

- Add Camera class with modes: vertical, horizontal, omni, floating
- Refactor scroll.py and demo to use Camera abstraction
- Add vis_offset for horizontal scrolling support
- Add camera_x to EffectContext for effects
- Add pygame window resize handling
- Add HUD effect plugin for demo mode
- Add --demo flag to run demo mode
- Add tests for Camera and vis_offset

tests/test_camera.py

@@ -0,0 +1,69 @@
+
+from engine.camera import Camera, CameraMode
+
+
+def test_camera_vertical_default():
+    """Test default vertical camera."""
+    cam = Camera()
+    assert cam.mode == CameraMode.VERTICAL
+    assert cam.x == 0
+    assert cam.y == 0
+
+
+def test_camera_vertical_factory():
+    """Test vertical factory method."""
+    cam = Camera.vertical(speed=2.0)
+    assert cam.mode == CameraMode.VERTICAL
+    assert cam.speed == 2.0
+
+
+def test_camera_horizontal():
+    """Test horizontal camera."""
+    cam = Camera.horizontal(speed=1.5)
+    assert cam.mode == CameraMode.HORIZONTAL
+    cam.update(1.0)
+    assert cam.x > 0
+
+
+def test_camera_omni():
+    """Test omnidirectional camera."""
+    cam = Camera.omni(speed=1.0)
+    assert cam.mode == CameraMode.OMNI
+    cam.update(1.0)
+    assert cam.x > 0
+    assert cam.y > 0
+
+
+def test_camera_floating():
+    """Test floating camera with sinusoidal motion."""
+    cam = Camera.floating(speed=1.0)
+    assert cam.mode == CameraMode.FLOATING
+    y_before = cam.y
+    cam.update(0.5)
+    y_after = cam.y
+    assert y_before != y_after
+
+
+def test_camera_reset():
+    """Test camera reset."""
+    cam = Camera.vertical()
+    cam.update(1.0)
+    assert cam.y > 0
+    cam.reset()
+    assert cam.x == 0
+    assert cam.y == 0
+
+
+def test_camera_custom_update():
+    """Test custom update function."""
+    call_count = 0
+
+    def custom_update(camera, dt):
+        nonlocal call_count
+        call_count += 1
+        camera.x += int(10 * dt)
+
+    cam = Camera.custom(custom_update)
+    cam.update(1.0)
+    assert call_count == 1
+    assert cam.x == 10

tests/test_layers.py

@@ -87,10 +87,26 @@ class TestRenderTickerZone:
 
     def test_returns_list(self):
         """Returns a list of strings."""
-        result, cache = layers.render_ticker_zone([], 0, 10, 80, {}, 0.0)
+        result, cache = layers.render_ticker_zone(
+            [],
+            scroll_cam=0,
+            camera_x=0,
+            ticker_h=10,
+            w=80,
+            noise_cache={},
+            grad_offset=0.0,
+        )
         assert isinstance(result, list)
 
     def test_returns_dict_for_cache(self):
         """Returns a dict for the noise cache."""
-        result, cache = layers.render_ticker_zone([], 0, 10, 80, {}, 0.0)
+        result, cache = layers.render_ticker_zone(
+            [],
+            scroll_cam=0,
+            camera_x=0,
+            ticker_h=10,
+            w=80,
+            noise_cache={},
+            grad_offset=0.0,
+        )
         assert isinstance(cache, dict)

tests/test_vis_offset.py

@@ -0,0 +1,32 @@
+
+from engine.effects.legacy import vis_offset, vis_trunc
+
+
+def test_vis_offset_no_change():
+    """vis_offset with offset 0 returns original."""
+    result = vis_offset("hello", 0)
+    assert result == "hello"
+
+
+def test_vis_offset_trims_start():
+    """vis_offset skips first N characters."""
+    result = vis_offset("hello world", 6)
+    assert result == "world"
+
+
+def test_vis_offset_handles_ansi():
+    """vis_offset handles ANSI codes correctly."""
+    result = vis_offset("\033[31mhello\033[0m", 3)
+    assert result == "lo\x1b[0m" or "lo" in result
+
+
+def test_vis_offset_greater_than_length():
+    """vis_offset with offset > length returns empty-ish."""
+    result = vis_offset("hi", 10)
+    assert result == ""
+
+
+def test_vis_trunc_still_works():
+    """Ensure vis_trunc still works after changes."""
+    result = vis_trunc("hello world", 5)
+    assert result == "hello"