Fix cylinder snake patterns indexing a lazy iterator

[SAY-5]

Fixes #80.

backandforth_pattern and frontandback_pattern build their index list with Iterators.flatten(...) and then return a closure that indexes into it. Base.Iterators.Flatten supports iteration but has no getindex, so calling the pattern (e.g. via linearize_index) throws MethodError: no method matching getindex(::Base.Iterators.Flatten{…}, ::Int64). Collecting the iterator before indexing resolves it.

In frontandback_pattern the even-L branch also used edge = L / 2, which is a Float64 and makes 1:edge a float range; switched both branches to integer division so the ranges stay integer-typed.

Added a testset that calls the helpers on an InfiniteCylinder and checks the result over one unit cell is a permutation of 1:L*N.