Test Linearity

Pushforwards and pullbacks are requred to be linear operators.
Which formally is defined as φ(𝛼x) = 𝛼φ(x) and φ(a+b) = φ(a) + φ(b) for a,b,x differentials/vector space elements, and 𝛼 being a scalar.

A important collory of that is φ(0) = 0 where two 0s are the additive identity for the relivent vector spaces.
Which we can test with iszero(φ(zero(d)), since we always have access to a differential d, and all differentials must define zero and iszero.

Similarly we can test φ(𝛼x) = 𝛼φ(x) via φ(2d) == 2φ(d) since all differnetial define multiplication with a scalar. and we have one provided d.

We can't easily test φ(a+b) = φ(a) + φ(b) though, as we only have one differential.
We can test φ(d+d) = φ(d) + φ(d) but that is less interesting.
If we are generating them using rand_tangent we can easily generate another.
Though perhaps we should be insisting that things define the addition of there natural differentials with the structural differentials that rand_tangent will most often provide (though sometimes rand_tangent's structural differential will have extra nondifferentiable fields so we can't trust it, unless we insist the user does actually overload it).
Or we could avoid direct additon and do (primal + rand_tangent(d)) + d) though this is getting off-topic..
Possibly just skip that in initial implementation of this

The linear pullback can be extracted from the rrule via (f,primal...) -> (ds...) -> last(rrule(f, primal...))(ds...) (or equiv: (f,primal) -> last(rrule(f, primal))) The linear pushforward can be extracted from frulevia(f,primal) -> (ds...) -> frule(ds..., f, primal...)`

No file or test path is named. Start by reading the existing rand_tangent, rrule, and frule test utilities, then determine how pushforward and pullback linearity should be checked. Done means adding agreed tests for zero and scalar behavior, with additivity included only if the available differential generation supports it.