Pullback random-attractor monitor

Apply this monitor to a stochastic RNN, state-space world model, or diffusion latent transition model during validation and optionally as a regularizer. The same noise realization must be replayed across different initial states; otherwise ordinary noise variability is confused with pullback attraction. Sample a noise block $\omega_{-n:-1}$ and $K$ initial states $z_{-n}^{(i)}$ from a fixed bounded ball. Run every state through the identical sequence of stochastic updates and compute the pairwise diameter, or an RMS approximation, after every step. Repeat for several noise blocks and record $D_{n,h}$. If a stable attractor is desired, add $L_{\mathrm{attr}}=\sum_{r=r_0}^{R}[\log D_{r+1,h}-\log D_{r,h}-\widehat\lambda_hh]_+^2$, where $[x]_+=\max(x,0)$. Trajectories and diameters are directly computed; the attractor is approximated by the terminal point cloud $A_{R,h}$; the decay exponent is fitted by regressing $\log D_{n,h}$ against $nh$. First experiment: use a two-dimensional noisy RNN on a synthetic damped oscillator with 32 initial states and 128-step histories, comparing Euler-Maruyama at several step sizes with a semi-implicit update. The prediction is exponential pullback contraction whose fitted slope matches the independently measured $\widehat\lambda_h$. Near the stability boundary, the contraction time should grow approximately as $1/|\lambda_h|$; a coarse solver with a spurious positive exponent should show persistent or increasing diameter.