docs(P1): align wording with level-list API only

Drop legacy constructor prose (child=/mutual exclusion), retarget
README and fixed_three_level_multipatch examples to refinements /
level_blocks, and fix stepper error/doc strings that still named
fine_block/base_block or a fixed gap >= 4 (unions use dependency
closure). Wording only; no behavior change.

src/CelerisLab/lbm/refinement_stepper.py

@@ -75,13 +75,13 @@ def _check_levels(refinements, blocks, objects, jets):
             raise ValueError(f"{name} needs one entry per level ({depth})")
     for level, block in enumerate(blocks):
         if block is not None and len(block) != 2:
-            raise ValueError(f"{_level_name(level)}_block must be (by, bx)")
+            raise ValueError(f"level_blocks[{level}] must be (by, bx)")
     if objects[0] or jets[0]:
         raise ValueError("base-level bodies/Jets are not supported")
     if any(blocks[level - 1] is not None and blocks[level] is None for level in range(1, depth)):
         raise ValueError("a blocked level requires blocked refined levels below it")
     if not is_rectangle(refinements[0]) and (blocks[0] is None or blocks[1] is None):
-        raise ValueError("several patches require fine_block and base_block")
+        raise ValueError("several L1 patches require level_blocks[0] and level_blocks[1]")
     if depth > 2:
         if any(block is None for block in blocks):
             raise ValueError("three or more levels must all be blocked")
@@ -149,9 +149,11 @@ class FixedPatchStepper:
     Nested levels: a child set's coordinates index the allocated parent array,
     including ghost bands and holes: root edge = parent fine_origin + index / 2
     (zero origin for a parent set; a scalar parent keeps its shifted window
-    origin). A scalar child requires a scalar parent. Each pair requires gap >= 4
-    in its own parent-cell units. All regions share one buffer pair per level
-    and the fixed schedule of 2**l hops per base step at level l. Bodies/Jets are supported only on the
+    origin). A scalar child requires a scalar parent. Rectangle unions impose no
+    fixed gap rule: adjacent, overlapping and concave unions are accepted when
+    their actual transfer dependencies close (FixedPatchSet2D /
+    nest_patch_set). All regions share one buffer pair per level and the fixed
+    schedule of 2**l hops per base step at level l. Bodies/Jets are supported only on the
     finest level of a nested hierarchy (L2 dx=dt=.25 in root units, unchanged
     lattice velocity); every whole circle and its BFL stencil must fit strictly
     away from that level's transfer footprint.