Add JAX backend with frame-varying random keys

- Add sexp_to_jax.py: JAX compiler for S-expression effects
- Use jax.random.fold_in for deterministic but varying random per frame
- Pass seed from recipe config through to JAX effects
- Fix NVENC detection to do actual encode test
- Add set_random_seed for deterministic Python random

The fold_in approach allows frame_num to be traced (not static)
while still producing different random patterns per frame,
fixing the interference pattern issue.

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

sexp_effects/primitive_libs/core.py

@@ -52,15 +52,33 @@ def prim_max(*args):
 
 
 def prim_round(x):
+    import numpy as np
+    if hasattr(x, '_data'):  # Xector
+        from .xector import Xector
+        return Xector(np.round(x._data), x._shape)
+    if isinstance(x, np.ndarray):
+        return np.round(x)
     return round(x)
 
 
 def prim_floor(x):
+    import numpy as np
+    if hasattr(x, '_data'):  # Xector
+        from .xector import Xector
+        return Xector(np.floor(x._data), x._shape)
+    if isinstance(x, np.ndarray):
+        return np.floor(x)
     import math
     return math.floor(x)
 
 
 def prim_ceil(x):
+    import numpy as np
+    if hasattr(x, '_data'):  # Xector
+        from .xector import Xector
+        return Xector(np.ceil(x._data), x._shape)
+    if isinstance(x, np.ndarray):
+        return np.ceil(x)
     import math
     return math.ceil(x)
 
@@ -193,6 +211,11 @@ def prim_range(*args):
 import random
 _rng = random.Random()
 
+def set_random_seed(seed):
+    """Set the random seed for deterministic output."""
+    global _rng
+    _rng = random.Random(seed)
+
 def prim_rand():
     """Return random float in [0, 1)."""
     return _rng.random()