Reject nonfinite or nonpositive circle geometry; tuple-normalize objects

_static_circles now checks finite center/radius and radius > 0 before
build_flag_mask (a negative radius squared to a one-cell solid), matching
prepare_fine_boundary. objects is converted to a tuple before the
emptiness test so an empty iterable takes the body-free path.
CPU tests: negative/NaN radius and nonfinite center rejected before CUDA;
empty iterator skips circle preparation.

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

src/CelerisLab/lbm/block_stepper.py

@@ -42,6 +42,10 @@ def _static_circles(layout, objects):
                 or any(getattr(obj.state, name) != 0 for name in ("vx", "vy", "omega"))
                 or any(vars(obj.control).values())):
             raise ValueError("Block bodies must be stationary passive circles")
+        cx, cy = obj.geometry.center
+        radius = obj.geometry._radius
+        if not np.isfinite([cx, cy, radius]).all() or radius <= 0:
+            raise ValueError("Block circle center/radius must be finite with radius > 0")
         mask = obj.geometry.build_flag_mask(nx, ny)
         solid = (mask & SOLID) != 0
         if not solid.any() or np.any(flags[solid] & SOLID):
@@ -93,7 +97,8 @@ class PeriodicBlockStepper:
         tpb = checked.threads_per_block
         if layout.nblocks > MAX_GRID_DIM or ly > MAX_GRID_DIM or not 0 < pairs < 2**31:
             raise ValueError("block count, block rows or halo pairs exceed launch limits")
-        self.boundary = _static_circles(layout, tuple(objects)) if objects else None
+        objects = tuple(objects)  # an empty iterable takes the body-free path
+        self.boundary = _static_circles(layout, objects) if objects else None
         links = 0 if self.boundary is None else len(self.boundary["cell"])
         if links >= 2**31:
             raise ValueError("cut links exceed launch limits")