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Dump verify_math.csv, move the superseded V=256 pilot CSVs to data/pilot.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

@@ -0,0 +1,10 @@
+check,claim,empirical,abs_err,tol,verdict
+V1 legit self-alignment,1.0,0.9994724071424732,0.000527592857526793,0.02,PASS
+V2a eve mean advantage,0.0,0.0006469982936097643,0.0006469982936097643,0.003,PASS
+V2b eve SER @ 0dB,0.99609375,0.995,0.0010937500000000044,0.015,PASS
+V2b eve SER @ 10dB,0.99609375,0.9888333333333333,0.007260416666666658,0.015,PASS
+V2b eve SER @ 20dB,0.99609375,0.9881666666666666,0.007927083333333362,0.015,PASS
+V2b eve SER @ 80dB,0.99609375,0.9896666666666667,0.0064270833333333055,0.015,PASS
+V3b random mask E|corr|,0.09973557010035818,0.10187042771408686,0.002134857613728683,0.009973557010035819,PASS
+V4a blind jammer projection mean,0.0,0.00026396107284673695,0.00026396107284673695,0.003,PASS
+V4b blind jammer projection variance,0.01558576233568703,0.015476787953278994,0.00010897438240803532,0.0007792881167843516,PASS