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/primitive_libs/core.py

@@ -39,6 +39,32 @@ def prim_mod(a, b):
     return a % b
 
 
+def prim_abs(x):
+    return abs(x)
+
+
+def prim_min(*args):
+    return min(args)
+
+
+def prim_max(*args):
+    return max(args)
+
+
+def prim_round(x):
+    return round(x)
+
+
+def prim_floor(x):
+    import math
+    return math.floor(x)
+
+
+def prim_ceil(x):
+    import math
+    return math.ceil(x)
+
+
 # Comparison
 def prim_lt(a, b):
     return a < b
@@ -98,6 +124,17 @@ def prim_get(obj, key, default=None):
     return default
 
 
+def prim_nth(seq, i):
+    i = int(i)
+    if 0 <= i < len(seq):
+        return seq[i]
+    return None
+
+
+def prim_first(seq):
+    return seq[0] if seq else None
+
+
 def prim_length(seq):
     return len(seq)
 
@@ -127,6 +164,31 @@ def prim_is_nil(x):
     return x is None
 
 
+# Higher-order / iteration
+def prim_reduce(seq, init, fn):
+    """(reduce seq init fn) — fold left: fn(fn(fn(init, s0), s1), s2) ..."""
+    acc = init
+    for item in seq:
+        acc = fn(acc, item)
+    return acc
+
+
+def prim_map(seq, fn):
+    """(map seq fn) — apply fn to each element, return new list."""
+    return [fn(item) for item in seq]
+
+
+def prim_range(*args):
+    """(range end), (range start end), or (range start end step) — integer range."""
+    if len(args) == 1:
+        return list(range(int(args[0])))
+    elif len(args) == 2:
+        return list(range(int(args[0]), int(args[1])))
+    elif len(args) >= 3:
+        return list(range(int(args[0]), int(args[1]), int(args[2])))
+    return []
+
+
 # Core primitives dict
 PRIMITIVES = {
     # Arithmetic
@@ -135,6 +197,12 @@ PRIMITIVES = {
     '*': prim_mul,
     '/': prim_div,
     'mod': prim_mod,
+    'abs': prim_abs,
+    'min': prim_min,
+    'max': prim_max,
+    'round': prim_round,
+    'floor': prim_floor,
+    'ceil': prim_ceil,
 
     # Comparison
     '<': prim_lt,
@@ -151,6 +219,8 @@ PRIMITIVES = {
 
     # Data access
     'get': prim_get,
+    'nth': prim_nth,
+    'first': prim_first,
     'length': prim_length,
     'len': prim_length,
     'list': prim_list,
@@ -161,4 +231,10 @@ PRIMITIVES = {
     'list?': prim_is_list,
     'dict?': prim_is_dict,
     'nil?': prim_is_nil,
+
+    # Higher-order / iteration
+    'reduce': prim_reduce,
+    'fold': prim_reduce,
+    'map': prim_map,
+    'range': prim_range,
 }