Learned-key stages, per-scheme plot styles, KM naming

exp_learned.py mirrors every structured result stage for the learned key
family at the same SNRs, frame counts and seeds, so Figs. 2, 3, 4 and 7
and Tables IV and VI can carry both realizations of keyed masking.

replot_security.py gains a style registry: colour identifies the scheme
and line style the role, so a curve learned in one figure reads the same
in the next. Previously OMA was grey in two figures and teal in a third,
and blue meant the eavesdropper in one figure and the permutation key in
another.

data/sec_compare.csv

@@ -4,3 +4,4 @@ public_mask,0.0529425,0.0529425,0.0529425,0.83882
 perm_key,0.0528525,0.9999925,0.0528525,0.36425
 index_cipher,0.0529425,0.9999847412,0.9999847412,0.83882
 oma_plain,0.08056383667,0.08056383667,0.08056383667,nan
+proposed_learned,0.06358416667,0.9998083333,0.9999833333,0.4046066667