FixedPatchStepper: level-indexed API, drop 15 fine/child compatibility views

- Hierarchy documents level/pair/shape/frame conventions; edge_frame(l)
  replaces the stepper's inline origin/spacing loop.
- Public per-level read-only state: layouts, boundaries, jets, leaf.
- set_jet_amplitude/read_raw_exchange/read_exchange_window: level= required.
- remap_child(child_objects=) -> remap_finest(objects=).
- _kernel_configs, _upload_boundary, _observed_boundary/_summarize split
  construction and observation duplication; behavior unchanged.
- fine_level_origin -> fine_level_offset (a (y, x) index, not an origin).
- docs: migration map in docs/development/src_coherence.md.

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

docs/synchronized_leaf_remap.md

@@ -33,7 +33,7 @@ origin is retained. Physical lower edges are `origin + spacing*(x,y)` on L0,
 `origin + spacing*(parent.fine_origin + (x,y)/4)` on L2.
 
 Factory options `coarse_boundary` and `objects` must declare `"periodic"` and
-an empty collection; L2 bodies use `child_objects` as described below.
+an empty collection; L2 bodies use `objects` as described below.
 LBMConfig's channel settings do not select this operator's boundary mode.
 Geometry metadata is factory-created;
 manual mutation/forging of metadata is unsupported. Caller certifies the
@@ -77,7 +77,7 @@ PYTHONPATH=src python -m unittest discover -s tests/unit -p 'test_synchronized_r
 
 ## Explicit runtime continuation
 
-`replacement = current.remap_child(rectangles)` returns a new ready
+`replacement = current.remap_finest(rectangles)` returns a new ready
 `FixedPatchStepper` at the **same absolute step_count**. It supports only
 periodic three-level runs with no L0/L1 bodies and an existing L2
 `FixedPatchSet2D`; stationary L2 bodies/Jets follow the support contract below.
@@ -99,7 +99,7 @@ resources; an actual old-device synchronization/read failure marks the old
 stepper failed, consistent with its existing failure semantics.
 
 ```python
-replacement = current.remap_child([(20, 16, 36, 40)])
+replacement = current.remap_finest([(20, 16, 36, 40)])
 # Explicitly choose the returned continuation only after success.
 current.close()
 current = replacement
@@ -119,7 +119,7 @@ Per-population conservation is not an advancement target under collision.
 
 ### Bounded pinned-body extension
 
-`replacement = current.remap_child(rectangles, child_objects=original_objects)`
+`replacement = current.remap_finest(rectangles, objects=original_objects)`
 also accepts stationary Circles inside the L2 union. Each body's **physical
 boundary, complete BFL support and current per-link velocity must remain
 identical**. Rectangles are not pinned: they may be split, merged or resized,