WGSL permits relaxed precision on pow, sin and
cos: an implementation may answer to a few ULP rather than correctly rounding.
The rotary embedding is the only part of the forward pass that calls them, and its kernel
comparison failed at 2.2e-5 against a 1e-5 bound. This measures each function separately,
over exactly the angles the rotary embedding uses, so the failure can be attributed instead
of guessed at.