* fix Muon weight decay being a no-op
LARS._step computed the post-momentum weight decayed param but only
used it for a dtype cast, so the decay was never applied. Fold the
decay into the update instead, matching torch's param.mul_(1 - lr*wd).
test_muon_wd passed anyway since lr*wd=1e-5 is far below atol, so
also bump the test's weight_decay to 10 to actually exercise it.
* muon: apply weight decay after lr scaling
keeps the decoupled decay independent of the LARS trust ratio r,
matching torch's param.mul_(1 - lr*wd). no behavior change today since
r is always 1.0 on the pre_wd=False path (Muon has tcoef=0).
