Add streaming video compositor with sexp interpreter

- New streaming/ module for real-time video processing:
  - compositor.py: Main streaming compositor with cycle-crossfade
  - sexp_executor.py: Executes compiled sexp recipes in real-time
  - sexp_interp.py: Full S-expression interpreter for SLICE_ON Lambda
  - recipe_adapter.py: Bridges recipes to streaming compositor
  - sources.py: Video source with ffmpeg streaming
  - audio.py: Real-time audio analysis (energy, beats)
  - output.py: Preview (mpv) and file output with audio muxing

- New templates/:
  - cycle-crossfade.sexp: Smooth zoom-based video cycling
  - process-pair.sexp: Dual-clip processing with effects

- Key features:
  - Videos cycle in input-videos order (not definition order)
  - Cumulative whole-spin rotation
  - Zero-weight sources skip processing
  - Live audio-reactive effects

- New effects: blend_multi for weighted layer compositing
- Updated primitives and interpreter for streaming compatibility

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

sexp_effects/interpreter.py

@@ -793,6 +793,35 @@ class Interpreter:
             return list(self.effects.values())[-1]
         return None
 
+    def load_effect_from_string(self, sexp_content: str, effect_name: str = None) -> EffectDefinition:
+        """Load an effect definition from an S-expression string.
+
+        Args:
+            sexp_content: The S-expression content as a string
+            effect_name: Optional name hint (used if effect doesn't define its own name)
+
+        Returns:
+            The loaded EffectDefinition
+        """
+        expr = parse(sexp_content)
+
+        # Handle multiple top-level expressions
+        if isinstance(expr, list) and expr and isinstance(expr[0], list):
+            for e in expr:
+                self.eval(e)
+        else:
+            self.eval(expr)
+
+        # Return the effect if we can find it by name
+        if effect_name and effect_name in self.effects:
+            return self.effects[effect_name]
+
+        # Return the most recently loaded effect
+        if self.effects:
+            return list(self.effects.values())[-1]
+
+        return None
+
     def run_effect(self, name: str, frame, params: Dict[str, Any],
                    state: Dict[str, Any]) -> tuple:
         """