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Dump verify_math.csv, move the superseded V=256 pilot CSVs to data/pilot.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_q3_jamming.csv

@@ -0,0 +1,8 @@
+jsr_db,ser_aligned,ser_random
+-10,0.1924166667,0.1780558333
+-5,0.2540891667,0.2948208333
+0,0.3063416667,0.4912158333
+5,0.3530991667,0.7022516667
+10,0.4284433333,0.8538791667
+15,0.5548433333,0.935385
+20,0.71272,0.9718341667