Add skip_update and magma support

[alint77]

Implements two stochastic update masking techniques from arxiv.org/abs/2602.15322 for both Muon and NorMuon.

SkipUpdate (skip_update_prob): at each step, each parameter matrix is independently kept with probability p or zeroed out with probability 1-p. Surviving updates are rescaled by 1/p to stay unbiased in expectation. Moment buffers always update densely regardless of the skip.

Magma (magma_tau): replaces the fixed 1/p rescaling with an adaptive EMA scale driven by momentum-gradient cosine similarity:

ẽ_t = sigmoid(cossim(μ_t_before, g_t) / τ)
s_t  = 0.9 * s_{t-1} + 0.1 * ẽ_t

The scale is intentionally biased (no 1/s_t correction), the paper found unbiased variants to be unstable. Bernoulli masking is still applied on top.

Both features are opt-in and off by default (None). For NorMuon, the mask is applied after the neuron-normalization step so both moment buffers (momentum and variance_neuron) always update densely, consistent with the paper's intent.

[alint77]

@microsoft-github-policy-service agree