fix: let sum_back stop at each group's edge, as its checks already assumed

[FBumann]

What this changes

sum_back accepts by=<lookup>, so a window stops at each group's edge instead of walking across it.

• operators.py — declares the kwarg on the sum_back builtin (lookup_kwargs, optional_kwargs) and shows it in the usage string.
• typesetting/walk.py — a new _group() renders the partition as the superscript its operator carries, and the window's lag passes it through.
• examples/operators/sum_back_partitioned.yaml, one golden constraint, one tools/spec_math.py row; docs and goldens regenerated from those.

Extracted from #59, which does the same for both windows. The sum_forward half stays there, since that operator only exists in #56.

Why

Two of the three places that describe this rule already agreed it held. The comment beside BUILTINS says by= partitions the axis for "shift and sum_back", and _dims_call already runs every partition check for both names — one lookup only, and it must be over the walked dim. Only BUILTINS never declared the kwarg, and that is the one that gates parsing, so the rule they described was unreachable:

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

Declaring the kwarg is the whole fix. The typesetting side needed the group as a superscript on the operator, which _Step.within and translation(step, group=...) were already shaped for — a partition rides a window exactly as it rides a leaf translation, and for the same reason: what the group changes is where the axis ends, not which coordinate is being written.

∑_{t' ∈ 𝒯 : 0 ≤ t -^season_of(t) t' < 3} on_{t',g} ≤ units_g   ∀ t ∈ 𝒯, g ∈ 𝒢

This is byte-identical to what #59 renders for the same constraint.

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📚 math-spec | 🛠️ Build #34218212 | 📁 Comparing 6e187d3 against latest (f1c207f)

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• examples/operators/index.html
• reference/notation/index.html
• reference/language/operators/index.html
• reference/math_spec/operators/index.html
• reference/math_spec/typesetting/walk/index.html