Add analytic and numerical MuonClip polar Jacobian baseline

[charlesmartin14]

Adds an MNIST MLP3 MuonClip baseline for the local polar-projection Jacobian at single matrix checkpoints.

For each layer matrix W, the notebook computes J = D Pi(W) and compares the Gram spectrum G = J*J in two ways: (1) analytically from the closed-form polar Frechet derivative and its exact eigenvalues 4/(sigma_i+sigma_j)^2 plus rectangular 1/sigma_i^2 modes, and (2) numerically from isotropic random perturbations, centered finite differences of Pi(W), and the empirical response Gram X^T X.

Both raw positive spectra are passed directly to WeightWatcher's WW_powerlaw.pl_fit. The notebook compares WeightWatcher alpha, KS D, xmin, and tail size, and includes a probe-count convergence helper for a single checkpoint. No spectrum normalization, trace normalization, log-binning, KDE, entropy, participation-ratio, or hand-written power-law fit is used.

The reusable operator code lives in baseline/rg_baselines/polar_jacobian.py; the experiment driver and mathematical derivation live in baseline/notebooks/MNIST_MLP3_MuonClip_Polar_Jacobian_Baseline.ipynb.