add mathematical spec for C32

doc/mathspec

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+Let siphash24 be the standard siphash-2-4 function [0]
+with a 256 bit key K=<k0,k1,k2,k3> instead of the 128-bit one,
+and a modified Initialization phase that sets v_i to k_i for 0 <= i < 4.
+
+Set N = 2^32
+Define a bipartite graph [1] G_K=(V,E) with N edges on N + N nodes as follows:
+  For 0 <= i < N, E_i = (V_i_0, V_i_1) = (siphash24(K,2*i) % N, siphash24(K,2*i+1) % N)
+
+Define the associated bipartite node-pairs graph G'_K=(V',E') with N edges on N/2 + N/2 node-pairs as follows:
+  For 0 <= i < N, E'_i = (V_i_0>>1, V_i_1>>1)
+
+A Cuckatoo32 solution for key K is a 42-cycle [2] in G'_K that is a matching [3] in G_K.
+
+For verification purposes, the solution is given as the sequence of 42 edge indices in increasing order.
+
+[0] https://cr.yp.to/siphash/siphash-20120918.pdf
+[1] https://en.wikipedia.org/wiki/Bipartite_graph
+[2] https://en.wikipedia.org/wiki/Cycle_(graph_theory)
+[3] https://en.wikipedia.org/wiki/Matching_(graph_theory)