Learned-key sensitivity, brute-force and real-token stages; legend order

exp_learned.py completes the learned side of the result stages, so every
figure can carry both realizations of keyed masking. real() holds the
structured artifacts aside and restores them, since exp_real_sec writes
fixed file names.

replot_security.py ranks legend handles from one declared order at all
three ax.legend call sites, so entries no longer follow plot-call order
and drift between figures.

data/sec_sens_learned.csv

@@ -0,0 +1,14 @@
+frac,ser_mask
+0,0.9999754167
+0.2,0.9977270833
+0.4,0.9502445833
+0.6,0.6829479167
+0.75,0.31458375
+0.85,0.1250175
+0.9,0.09168625
+0.92,0.08508416667
+0.94,0.07654291667
+0.955,0.0716375
+0.97,0.06938458333
+0.985,0.06607833333
+1,0.0636775