feat: add lazy register_pytree_node helpers for opt-in JAX pytree registration

Library code can now register classes as JAX pytree nodes without forcing
jax.tree_util to import at module load. Both helpers no-op if JAX is missing,
so they're safe to call from code paths that may run without JAX installed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 2 people

autoconf/jax_wrapper.py

@@ -47,8 +47,40 @@
         """"
         JAX 64-bit precision has been automatically enabled for you (JAX_ENABLE_X64=True),
         as double precision is required for most scientific computing applications.
-        
-        To enable 64 precision as default in JAX, set the environment variable 
+
+        To enable 64 precision as default in JAX, set the environment variable
         JAX_ENABLE_X64=true before running your script.
         """
     )
+
+
+def register_pytree_node_class(cls):
+    """Opt-in JAX pytree class registration that defers the JAX import.
+
+    The previous eager registration in ``autofit.mapper.prior_model.prior_model``
+    forced ``jax.tree_util`` to load whenever ``import autofit`` ran. To keep
+    JAX an optional dependency, library code now exposes ``tree_flatten`` /
+    ``tree_unflatten`` methods but does NOT register the class itself; callers
+    that want JAX integration call this helper explicitly (typically via
+    ``autofit.jax.enable_pytrees()``).
+
+    No-ops if JAX is not installed.
+    """
+    try:
+        from jax.tree_util import register_pytree_node_class as _r
+    except ImportError:
+        return cls
+    return _r(cls)
+
+
+def register_pytree_node(nodetype, flatten_func, unflatten_func):
+    """Opt-in JAX pytree registration for an externally-defined class.
+
+    Lazy counterpart to :func:`register_pytree_node_class` for the case where
+    the class cannot be decorated directly. No-ops if JAX is not installed.
+    """
+    try:
+        from jax.tree_util import register_pytree_node as _r
+    except ImportError:
+        return None
+    return _r(nodetype, flatten_func, unflatten_func)