P1 (12/12): one FixedPatchStepper construction and initialize API

Removes the legacy entry points now that every caller is migrated:
FixedPatchStepper(cfg, *, context, refinements, coarse_boundary,
level_blocks, level_objects, level_jets, observe_*, explosion) and
initialize(*level_g). Deleted: positional bounds, fine_block, base_block,
fine_objects, jets, child, child_block, child_objects, child_jets,
child_rectangles_in_parent, initialize(coarse_g/fine_g/child_g),
_legacy_levels and _initial_levels. Docstring, README and docs describe
the level lists. Compatibility views (patch, fine_boundary, ...) and the
PeriodicPatchStepper alias are unchanged (separate follow-up).

Error -> rule table (legacy keyword check -> level-list rule now firing):
- several patches without fine_block/base_block -> _check_levels
  'several patches require fine_block and base_block' (unchanged rule)
- base_block without fine_block -> 'a blocked level requires blocked
  refined levels below it'
- nested child without all blocks -> 'three or more levels must all be
  blocked' / 'a blocked level requires ...'
- fine_objects/jets with a nested child ('only on L2') -> 'bodies/Jets are
  supported only on the finest level of a nested hierarchy'
- child_objects/child_jets without a nested child -> 'level_objects/
  level_jets needs one entry per level'
- scalar child with several L1 patches ('scalar child requires one parent
  patch') -> hierarchy 'a scalar child requires a scalar parent'
- child vs child_rectangles_in_parent mutually exclusive -> no level-list
  equivalent (cannot be expressed); covered by the new explicit
  RemovedLegacyAPI test: removed keywords and positional bounds raise
  TypeError before any context use, initialize has only *level_g.
The InitialLevels test of the removed adapter is replaced by that test.

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

docs/fixed_three_level_multipatch.md

@@ -16,8 +16,8 @@ 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.
 
-The new keyword `child_rectangles_in_parent=[...]` is mutually exclusive with
-`child`. It accepts a scalar or a set parent and interprets every rectangle in
+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`:
@@ -47,9 +47,9 @@ tile edges.
 
 ```python
 run = FixedPatchStepper(
-    cfg, [(4, 4, 20, 28), (24, 4, 44, 28)], context=ctx,
-    base_block=(4, 4), fine_block=(4, 4), child_block=(4, 4),
-    child_rectangles_in_parent=[(16, 16, 32, 40), (56, 16, 80, 40)],
+    cfg, context=ctx,
+    refinements=[[(4, 4, 20, 28), (24, 4, 44, 28)], [(16, 16, 32, 40), (56, 16, 80, 40)]],
+    level_blocks=[(4, 4)] * 3,
 )
 ```
 
@@ -57,7 +57,7 @@ This example uses a 48x32 root. `initialize` and `download` use three Q-major
 dense arrays. Public arrays and masks still scale with the full allocated
 extent; sparse device buffers do not remove that host-memory limit.
 
-Stationary whole-circle `child_objects` and `child_jets` use root edge
+Stationary whole-circle L2 bodies and Jets (`level_objects[2]`, `level_jets[2]`) use root edge
 coordinates and a single L2 boundary object. Body IDs are physical object-list
 positions, not region IDs or caller `obj_id`. Complete boundary stencils must
 avoid the transfer footprint. Change a child Jet with