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.

docs/fixed_three_level_multipatch.md

@@ -1,8 +1,8 @@
 # Fixed three-level rectangle unions
 
-This page describes the legacy three-level entry point of `FixedPatchStepper`,
-with rectangle unions on either adjacent pair. The `refinements` level-list
-entry point also supports arbitrary depth; three or more levels require
+This page describes three-level `FixedPatchStepper` setups built from the
+`refinements` level list, with rectangle unions on either adjacent pair. The
+same constructor supports arbitrary depth; three or more levels require
 all-blocked storage and bodies/Jets only on the finest level. This is not
 general AMR: there is no per-region timestep, moving geometry, automatic
 dynamic refinement or L1 body support in the three-level form.
@@ -12,15 +12,15 @@ No CUDA kernel changes are involved.
 
 ## Explicit coordinate frames
 
-Keep the existing scalar `child=(x0,y0,x1,y1)` for a single scalar parent patch.
-Its coordinates retain their original meaning: half-open integer indices in the
-allocated L1 window, including its ghost band.
+For a single scalar parent patch, pass a scalar rectangle as `refinements[1]`:
+`(x0,y0,x1,y1)` in half-open integer indices of the allocated L1 window,
+including its ghost band.
 
-A rectangle list as the second `refinements` entry (formerly the keyword
-`child_rectangles_in_parent=[...]`) accepts a scalar or a set parent and interprets every rectangle in
-the actual allocated L1 array, including ghost bands and holes. It never infers
-region-local coordinates. The physical root cell edge corresponding to an L1
-index `(x,y)` is `parent.fine_origin + (x,y)/2`:
+A rectangle list as `refinements[1]` accepts a scalar or a set parent and
+interprets every rectangle in the actual allocated L1 array, including ghost
+bands and holes. It never infers region-local coordinates. The physical root
+cell edge corresponding to an L1 index `(x,y)` is
+`parent.fine_origin + (x,y)/2`:
 
 - A scalar L1 parent has a shifted window origin.
 - A set L1 parent has origin `(0,0)` and public shape `(2*ny,2*nx)`.