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>

effects/ascii_dual_blend.sexp

@@ -19,6 +19,8 @@
       :desc "Blend opacity (0=video-a only, 1=video-b only)")
     (blend_mode :type string :default "overlay"
       :desc "Blend mode: alpha, add, multiply, screen, overlay, difference")
+    (duration :type float :default 10 :range [1 300]
+      :desc "Clip duration in seconds")
   )
 
   ;; Registry - effects and analyzers
@@ -35,7 +37,7 @@
   ;; Stage 1: Analysis
   (stage :analyze
     :outputs [energy-data]
-    (def audio-clip (-> audio (segment :start 60 :duration 10)))
+    (def audio-clip (-> audio (segment :start 60 :duration duration)))
     (def energy-data (-> audio-clip (analyze energy))))
 
   ;; Stage 2: Process both videos
@@ -45,10 +47,10 @@
     :outputs [blended audio-clip]
 
     ;; Get audio clip for final mux
-    (def audio-clip (-> audio (segment :start 60 :duration 10)))
+    (def audio-clip (-> audio (segment :start 60 :duration duration)))
 
     ;; Process video A with ASCII effect
-    (def clip-a (-> video-a (segment :start 0 :duration 10)))
+    (def clip-a (-> video-a (segment :start 0 :duration duration)))
     (def ascii-a (-> clip-a
       (effect ascii_fx_zone
         :cols cols
@@ -66,7 +68,7 @@
                             (- 1 (get zone "row-norm")))))))))))
 
     ;; Process video B with ASCII effect
-    (def clip-b (-> video-b (segment :start 0 :duration 10)))
+    (def clip-b (-> video-b (segment :start 0 :duration duration)))
     (def ascii-b (-> clip-b
       (effect ascii_fx_zone
         :cols cols