Add the key-refresh layer

Derives each block key from the transformations that leave the decision
statistic invariant: a global sign per frame entry, a relabeling of the
frame entries applied to keys and codebook together, and a permutation
of which user holds which row. The legitimate error rate is unchanged
at 0.257 across 24 blocks while a refresh that installs fresh
orthogonal keys reaches 0.710, and a key recovered by known plaintext
returns to the random-guess level one block later.

data/refresh_kpa.csv

@@ -0,0 +1,7 @@
+n_frames,ser_same_block,ser_next_block
+2,0.2777190625,0.998930625
+4,0.2615640625,0.998644375
+8,0.2594196875,0.998631875
+16,0.2582471875,0.99867
+32,0.2579625,0.99871875
+64,0.257455,0.9987190625