refactor: move effects_plugins to engine/effects/plugins

- Move effects_plugins/ to engine/effects/plugins/
- Update imports in engine/app.py
- Update imports in all test files
- Follows capability-based deps architecture

Closes #27

engine/effects/plugins/crop.py

@@ -0,0 +1,42 @@
+from engine.effects.types import EffectConfig, EffectContext, EffectPlugin
+
+
+class CropEffect(EffectPlugin):
+    """Crop effect that crops the input buffer to fit the display.
+
+    This ensures the output buffer matches the actual display dimensions,
+    useful when the source produces a buffer larger than the viewport.
+    """
+
+    name = "crop"
+    config = EffectConfig(enabled=True, intensity=1.0)
+
+    def process(self, buf: list[str], ctx: EffectContext) -> list[str]:
+        if not buf:
+            return buf
+
+        # Get actual display dimensions from context
+        w = (
+            ctx.terminal_width
+            if ctx.terminal_width > 0
+            else max(len(line) for line in buf)
+        )
+        h = ctx.terminal_height if ctx.terminal_height > 0 else len(buf)
+
+        # Crop buffer to fit
+        result = []
+        for i in range(min(h, len(buf))):
+            line = buf[i]
+            if len(line) > w:
+                result.append(line[:w])
+            else:
+                result.append(line + " " * (w - len(line)))
+
+        # Pad with empty lines if needed
+        while len(result) < h:
+            result.append(" " * w)
+
+        return result
+
+    def configure(self, config: EffectConfig) -> None:
+        self.config = config