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_keylen_learned.csv

@@ -0,0 +1,9 @@
+L,d,legit_ser,eve_ser,mask_xcorr,oma
+8,32,0.9297855,0.9998735,0.007307400461,0.6849191155
+12,48,0.416604,0.9997065,0.005153660662,nan
+16,64,0.2762895,0.999912,0.007116591092,0.2747696909
+20,80,0.2076175,0.999383,0.003162040841,0.2289444229
+24,96,0.1829615,0.999894,0.002973971656,0.1961714033
+32,128,0.131901,0.999616,0.005575809628,0.1524639978
+48,192,0.090206,0.9994575,0.005743456539,0.1054308944
+64,256,0.0635265,0.999637,0.006678360514,0.08056383667