Docs: note where library defaults diverge from the arXiv 2607.20548 recipe (momentum, weight decay, NS iterations, RMS scale)

[adpena]

The recent paper "SOAP, Muon, and Beyond" (arXiv 2607.20548) points readers to this repository as its released code. Readers who install the library and use Muon's constructor defaults get a materially different optimizer than the paper's experiments on four axes:

axis	paper recipe	library default (muon.py @ HEAD)
momentum	0.9	0.95
decoupled weight decay	0.1	0.01
orthogonalization	16-step PolarExpress	5-step quintic
update-RMS correction	~0.2 (sqrt((1-b1)/(1+b1)))	extra_scale_factor=1.0 (off)

Since the paper itself argues optimizer comparisons are uninterpretable without matched update RMS, a short README note or a checked-in "paper recipe" reference config would prevent exactly the silent config mismatch the paper warns about. The only current breadcrumb is the extra_scale_factor docstring's optional 0.2 mention, which cites arXiv 2502.16982 rather than the recipe. Happy to PR the table above into the README if that's welcome.

[skyw]

Thanks for noticing this.

Changing default values in code can break some other code and complicate backward combability. We'll think about how to deal with it.