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>

streaming/output.py

@@ -37,19 +37,39 @@ _nvenc_available: Optional[bool] = None
 
 
 def check_nvenc_available() -> bool:
-    """Check if NVENC hardware encoding is available."""
+    """Check if NVENC hardware encoding is available and working.
+
+    Does a real encode test to catch cases where nvenc is listed
+    but CUDA libraries aren't loaded.
+    """
     global _nvenc_available
     if _nvenc_available is not None:
         return _nvenc_available
 
     try:
+        # First check if encoder is listed
         result = subprocess.run(
             ["ffmpeg", "-encoders"],
             capture_output=True,
             text=True,
             timeout=5
         )
-        _nvenc_available = "h264_nvenc" in result.stdout
+        if "h264_nvenc" not in result.stdout:
+            _nvenc_available = False
+            return _nvenc_available
+
+        # Actually try to encode a small test frame
+        result = subprocess.run(
+            ["ffmpeg", "-y", "-f", "lavfi", "-i", "testsrc=duration=0.1:size=64x64:rate=1",
+             "-c:v", "h264_nvenc", "-f", "null", "-"],
+            capture_output=True,
+            text=True,
+            timeout=10
+        )
+        _nvenc_available = result.returncode == 0
+        if not _nvenc_available:
+            import sys
+            print("NVENC listed but not working, falling back to libx264", file=sys.stderr)
     except Exception:
         _nvenc_available = False