`sum_back`/`sum_forward` partition-checking is unreachable, and the docstring claims `by=` works

[FBumann]

What happened?

dimensions.py runs full partition-checking for the windows:

if node.name in ('shift', 'sum_back', 'sum_forward'):
    ...
    partition = node.kwargs.get('by')
    if partition is not None:
        # two carefully-worded DimensionErrors about partitioning

Neither window's Builtin has lookup_kwargs, so call_shape_error rejects by= first and that code can never run:

constraints:
  c:
    foreach: [unit, hour]
    expression: sum_back(x, over=hour, within=3, by=day_of) <= 1

SchemaError: Constraint 'c': sum_back() expects sum_back(<expr>, over=<dim>, within=<n|parameter>[, edge='wrap'])

The module docstring in operators.py also states the opposite of what the signature allows:

by= on shift and sum_back partitions the axis the operator walks, which is the same lookup in a different position

What it should be

One of three, and the choice is a design decision rather than a cleanup:

1. Support it — add lookup_kwargs=('by',)/optional_kwargs=('by',) to both windows and the existing checks light up. "The next n hours inside this representative day" is a real ask, and it is the same by= as shift's.
2. Refuse it — drop the dead branch and correct the docstring.
3. Support it on neither and say why in the docstring.

Worth settling for the pair at once rather than twice. Whichever way it goes, the docstring and the signature should agree.

Version

v0.0.0-alpha.10 (and sum_forward inherits it — #56)

[FBumann]

Fixed on main by #65.

• sum_back(x, over=hour, within=3, by=day_of) loads and renders — the partition checks in _dims_call that nothing could reach are reachable now.
• The BUILTINS docstring saying by= works on "shift and sum_back" is true rather than aspirational, so signature and prose agree.

The title also names sum_forward, which did not exist when this was filed. It arrives in #56 with by= already wired and the docstring reworded to "the two windows", so the operator lands complete rather than acquiring a partition later. Nothing outstanding here either way.