Audit round: fair OMA reference, dense grids, covariance-attack checks

Resource-match the OMA reference in the key-length sweep (oma_ser_keylen),
which gives it the L/16 combining gain the longer frame allows. The
proposal now passes a resource-matched OMA by 1.27x at L=64 rather than
the 4.3x reported against a fixed-d reference.

Densify the JSR, sensitivity, and brute-force grids so the curves are
smooth, give the index cipher its channel floor instead of error-free
reception, and add the permutation-key known-plaintext attack
(exp_permkpa) so Fig. 7 carries a conventional linear scheme.

Add check_cov_attack.py and check_cov_ceiling.py: a referee raised a
ciphertext-only second-order attack; the exact-population test shows the
received covariance leaks only a sparse rank-deficient subset of the key
Gram and leaves the eavesdropper at the random-guess level.

Dump verify_math.csv, move the superseded V=256 pilot CSVs to data/pilot.

data/pkpa.csv

@@ -0,0 +1,15 @@
+n_frames,perm_frac,eve_ser
+1,0.20234375,0.998977
+2,0.7140625,0.7107255
+3,0.91484375,0.4172725
+4,0.94921875,0.324415
+5,0.9515625,0.340107
+6,0.94765625,0.3034795
+8,0.95625,0.303056
+10,0.95546875,0.3027505
+12,0.94921875,0.3028
+16,0.9546875,0.3036285
+24,0.94921875,0.303151
+32,0.95234375,0.302631
+48,0.95,0.3030415
+64,0.9515625,0.3025125