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Dump verify_math.csv, move the superseded V=256 pilot CSVs to data/pilot.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/pilot/feas_q1_eavesdrop.csv

@@ -0,0 +1,6 @@
+snr_db,legit,eve_wrong,eve_none,eve_avg,chance
+0,0.55692125,0.995261875,0.992021875,0.95962875,0.99609375
+5,0.26456,0.9946975,0.990336875,0.94351625,0.99609375
+10,0.099379375,0.994283125,0.989521875,0.93188,0.99609375
+15,0.0334,0.994011875,0.989031875,0.925765625,0.99609375
+20,0.010784375,0.9939975,0.988625625,0.922928125,0.99609375