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Dump verify_math.csv, move the superseded V=256 pilot CSVs to data/pilot.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_q2_keyentropy.csv

@@ -0,0 +1,6 @@
+L,d,legit_ser,eve_ser,mask_xcorr
+8,16,0.5546,0.9975366667,0.07264538854
+16,32,0.1181416667,0.99724,0.006702715065
+32,64,0.09873416667,0.9943241667,0.01185981557
+64,128,0.08845333333,0.9922383333,0.009370678104
+128,256,0.08771083333,0.9851966667,0.008488235064