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_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"