Kernels cannot declare their own type/const generics

`kernel.rs:148` rejects every non-lifetime generic parameter on a `#[kernel]` fn:

```
#[kernel] fns cannot declare their own type/const generics
(the macro owns `const N: usize` and the exec parameter)
```

`transform_sig` then pushes `const N: usize` and `__E` onto the parameter list,
and call sites are emitted as `::<N, _>` — positional. A user parameter list
would collide with that.
