write MidenVM constraints for system (clk constraint)

Most of the system constraints depend on the stack and are therefore more challenging and should be done separately, but the `clk` constraint can be done first. It's implemented in Rust [here](https://github.com/0xPolygonMiden/miden-vm/blob/main/air/src/lib.rs).

It's possible that this constraint is not documented in the Miden VM design docs - I didn't find it. It would be good to double-check this and also update the MidenVM docs to correct this.

- [ ] add `clk` constraint
- [ ] search the MidenVM design docs for the `clk` constraint and update the docs if not found