Add perturbation correlation audit (round 19's recommended alt metric)

Codex round 19 said: 'use nudging or perturbation correlation on the
penalized checkpoints. In the healthy-gradient regime, that is a more
direct is-the-local-signal-useful test than cosine alone'.

Result on existing checkpoints (eps=1e-3, M=32 random directions, n=1024):

  vanilla DFA s42:                    deep rho +0.002
  penalized DFA s42 lam=1e-2 30ep:    deep rho +0.094
  penalized DFA s123 lam=1e-2 30ep:   deep rho +0.073
  penalized DFA s456 lam=1e-2 30ep:   deep rho +0.072
  penalized 3-seed mean:              deep rho +0.080 ± 0.011

This INDEPENDENTLY TRIANGULATES the cos +0.17 finding via a different
metric:
  - vanilla deep cos ~0   matches  vanilla deep rho ~0
  - penalized deep cos +0.155  matches  penalized deep rho +0.080

The two metrics measure different things:
  - cos = directional alignment with BP grad
  - rho = correlation between predicted and true loss change under
    random perturbation

Both show the same pattern: penalty creates partial usefulness from
essentially zero. This is the 6th independent validation of the mode 2
'penalty creates partial alignment' framing.

Crucially, rho doesn't use F.cosine_similarity (no eps clamp), and it
measures sample-level loss change correlation rather than direction
match — so it rules out 'cos is capturing some directional artifact
unrelated to local usefulness'.

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