fix numba stiching blend error and add numpy blend fallback

src/orisys_blend/__init__.py

@@ -0,0 +1,10 @@
+"""Pure-Python Numba stitch kernels kept outside the Nuitka-compiled orisys .pyd.
+
+Nuitka cannot reliably JIT ``@njit`` functions baked into a compiled extension.
+This package is shipped as source next to the Nuitka wheel so Numba can see
+real CPython bytecode at runtime.
+"""
+
+from .kernel import blend_kernel
+
+__all__ = ["blend_kernel"]