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>

src/CelerisLab/lbm/refinement.py

@@ -237,7 +237,11 @@ class FixedPatchSet2D:
     def fine_layout(self, block):
         """Sparse non-periodic layout storing exactly the patch supports."""
         from .blocks import BlockLayout2D
-        by, bx = block
+        block = tuple(block)
+        if len(block) != 2 or any(isinstance(v, (bool, np.bool_)) or not isinstance(v, Integral)
+                                  or v <= 0 for v in block):
+            raise ValueError("fine block must be two positive integers (by, bx)")
+        by, bx = map(int, block)
         origins = set()
         for p in self.patches:
             y0, x0 = p.fine_level_origin