Multipatch: validate fine block before modulo, raw-core GPU checks, README usage

FixedPatchSet2D.fine_layout rejects non-positive, float, bool or wrong-length
block sizes with ValueError before any modulo (was ZeroDivisionError/TypeError).
The face gap-4 GPU test now checks raw owner cores directly by kind/covered
for every patch (ACTIVE finite, OUTER/holes/covered NaN; INNER not asserted),
not only the masked download and reversed-order parity. README documents the
body-free multipatch call, scope and level-wide fine shape.

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

tests/unit/test_refinement.py

@@ -228,6 +228,9 @@ class FixedPatchSetTests(unittest.TestCase):
             self.assertTrue(layout.owned.ravel()[s.ghost_fine.astype(np.intp)].all())
         with self.assertRaisesRegex(ValueError, "aligned"):
             self.build(self.FACE).fine_layout((5, 4))
+        for block in ((0, 4), (4, 0), (-4, 4), (4.0, 4), (True, 4), (4,), (4, 4, 4), (np.bool_(1), 4)):
+            with self.subTest(block=block), self.assertRaisesRegex(ValueError, "positive integers"):
+                self.build(self.FACE).fine_layout(block)
 
 
 if __name__ == "__main__":