Accept legacy named arrays in FixedPatchStepper.initialize

initialize(coarse_g=..., fine_g=..., child_g=...) raised TypeError after the
switch to *level_g. A small entry adapter (_initial_levels) maps either the
positional per-level form or the original names to one per-level tuple and
rejects mixed or incomplete forms with TypeError before any state check.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/CelerisLab/lbm/refinement_stepper.py

@@ -90,6 +90,19 @@ def _legacy_levels(bounds, *, fine_block, base_block, child, child_block, child_
     return refinements, blocks, objects, level_jets
 
 
+def _initial_levels(level_g, coarse_g, fine_g, child_g):
+    """One tuple of per-level arrays from initialize()'s positional or legacy named form."""
+    if coarse_g is None and fine_g is None:
+        if child_g is not None and len(level_g) != 2:
+            raise TypeError("child_g needs exactly the coarse and fine arrays before it")
+        return tuple(level_g) + (() if child_g is None else (child_g,))
+    if level_g:
+        raise TypeError("pass level arrays positionally or as coarse_g/fine_g/child_g, not both")
+    if coarse_g is None or fine_g is None:
+        raise TypeError("coarse_g and fine_g are both required in the named form")
+    return (coarse_g, fine_g) + (() if child_g is None else (child_g,))
+
+
 def _check_levels(refinements, blocks, objects, jets):
     """Candidate limits shared by the list and the legacy entry points."""
     depth = len(refinements) + 1
@@ -430,11 +443,15 @@ class FixedPatchStepper:
         for array, leaf in zip(arrays, self._leaf):
             array[:, ~leaf] = np.nan
 
-    def initialize(self, *level_g, child_g=None):
+    def initialize(self, *level_g, coarse_g=None, fine_g=None, child_g=None):
-        """Upload an explicit G_0, one Q-major array per level; non-leaves are discarded."""
+        """Upload an explicit G_0, one Q-major array per level; non-leaves are discarded.
+
+        Pass the level arrays positionally (L0 first), or use the original
+        two/three-level names coarse_g, fine_g[, child_g]; mixing is rejected.
+        """
+        given = _initial_levels(level_g, coarse_g, fine_g, child_g)
         with self._lock, self.ctx.activate():
             self._require("new", "ready")
-            given = level_g + (() if child_g is None else (child_g,))
             if len(given) != len(self._levels):
                 raise ValueError("Expected one Q-major array per level")
             arrays = [np.array(a, dtype=np.float32, order="C", copy=True) for a in given]