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>

streaming/__init__.py

@@ -0,0 +1,44 @@
+"""
+Streaming video compositor for real-time effect processing.
+
+This module provides a frame-by-frame streaming architecture that:
+- Reads from multiple video sources with automatic looping
+- Applies effects inline (no intermediate files)
+- Composites layers with time-varying weights
+- Outputs to display, file, or stream
+
+Usage:
+    from streaming import StreamingCompositor, VideoSource, AudioAnalyzer
+
+    compositor = StreamingCompositor(
+        sources=["video1.mp4", "video2.mp4"],
+        effects_per_source=[...],
+        compositor_config={...},
+    )
+
+    # With live audio
+    audio = AudioAnalyzer(device=0)
+    compositor.run(output="output.mp4", duration=60, audio=audio)
+
+    # With preview window
+    compositor.run(output="preview", duration=60)
+
+Backends:
+    - numpy: Works everywhere, ~3-5 fps (default)
+    - glsl: Requires GPU, 30+ fps real-time (future)
+"""
+
+from .sources import VideoSource, ImageSource
+from .compositor import StreamingCompositor
+from .backends import NumpyBackend, get_backend
+from .output import DisplayOutput, FileOutput
+
+__all__ = [
+    "StreamingCompositor",
+    "VideoSource",
+    "ImageSource",
+    "NumpyBackend",
+    "get_backend",
+    "DisplayOutput",
+    "FileOutput",
+]