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/effects/ascii_art.sexp

@@ -1,4 +1,5 @@
 ;; ASCII Art effect - converts image to ASCII characters
+(require-primitives "ascii")
 
 (define-effect ascii_art
   :params (

sexp_effects/effects/ascii_art_fx.sexp

@@ -1,4 +1,5 @@
 ;; ASCII Art FX - converts image to ASCII characters with per-character effects
+(require-primitives "ascii")
 
 (define-effect ascii_art_fx
   :params (

sexp_effects/effects/ascii_zones.sexp

@@ -1,5 +1,6 @@
 ;; ASCII Zones effect - different character sets for different brightness zones
 ;; Dark areas use simple chars, mid uses standard, bright uses blocks
+(require-primitives "ascii")
 
 (define-effect ascii_zones
   :params (

sexp_effects/effects/blend.sexp

@@ -15,7 +15,7 @@
     (opacity :type float :default 0.5)
     (resize_mode :type string :default "fit")
     (priority :type string :default "width")
-    (pad_color :type list :default [0 0 0])
+    (pad_color :type list :default (quote [0 0 0]))
   )
   (let [a frame-a
         a-w (width a)

sexp_effects/effects/blend_multi.sexp

@@ -0,0 +1,57 @@
+;; N-way weighted blend effect
+;;
+;; Takes N input frames via `inputs` and N per-frame weights.
+;; Produces a single frame: the normalised weighted composite.
+;;
+;; Parameters:
+;;   weights     - list of N floats, one per input (resolved per-frame)
+;;   mode        - blend mode applied when folding each frame in:
+;;                   "alpha"       — pure weighted average (default)
+;;                   "multiply"    — darken by multiplication
+;;                   "screen"      — lighten (inverse multiply)
+;;                   "overlay"     — contrast-boosting midtone blend
+;;                   "soft-light"  — gentle dodge/burn
+;;                   "hard-light"  — strong dodge/burn
+;;                   "color-dodge" — brightens towards white
+;;                   "color-burn"  — darkens towards black
+;;                   "difference"  — absolute pixel difference
+;;                   "exclusion"   — softer difference
+;;                   "add"         — additive (clamped)
+;;                   "subtract"    — subtractive (clamped)
+;;                   "darken"      — per-pixel minimum
+;;                   "lighten"     — per-pixel maximum
+;;   resize_mode - how to match frame dimensions (fit, crop, stretch)
+;;
+;; Uses a left-fold over inputs[1..N-1].  At each step the running
+;; opacity is:  w[i] / (w[0] + w[1] + ... + w[i])
+;; which produces the correct normalised weighted result.
+
+(require-primitives "image" "blending")
+
+(define-effect blend_multi
+  :params (
+    (weights :type list :default (quote []))
+    (mode :type string :default "alpha")
+    (resize_mode :type string :default "fit")
+  )
+  (let [n (len inputs)
+        ;; Target dimensions from first frame
+        target-w (width (nth inputs 0))
+        target-h (height (nth inputs 0))
+        ;; Fold over indices 1..n-1
+        ;; Accumulator is (list blended-frame running-weight-sum)
+        seed (list (nth inputs 0) (nth weights 0))
+        result (reduce (range 1 n) seed
+                 (lambda (pair i)
+                   (let [acc (nth pair 0)
+                         running (nth pair 1)
+                         w (nth weights i)
+                         new-running (+ running w)
+                         opacity (/ w (max new-running 0.001))
+                         f (resize (nth inputs i) target-w target-h "linear")
+                         ;; Apply blend mode then mix with opacity
+                         blended (if (= mode "alpha")
+                                   (blend-images acc f opacity)
+                                   (blend-images acc (blend-mode acc f mode) opacity))]
+                     (list blended new-running))))]
+    (nth result 0)))

sexp_effects/effects/invert.sexp

@@ -1,8 +1,9 @@
 ;; Invert effect - inverts all colors
 ;; Uses vectorized invert-img primitive for fast processing
+;; amount param: 0 = no invert, 1 = full invert (threshold at 0.5)
 
 (require-primitives "color_ops")
 
 (define-effect invert
-  :params ()
-  (invert-img frame))
+  :params ((amount :type float :default 1 :range [0 1]))
+  (if (> amount 0.5) (invert-img frame) frame))

sexp_effects/effects/ripple.sexp

@@ -1,4 +1,5 @@
 ;; Ripple effect - radial wave distortion from center
+(require-primitives "geometry" "image" "math")
 
 (define-effect ripple
   :params (

sexp_effects/effects/zoom.sexp

@@ -1,4 +1,5 @@
 ;; Zoom effect - zooms in/out from center
+(require-primitives "geometry")
 
 (define-effect zoom
   :params (