vsr: assert that view headers have correct command on the write path

[matklad]

In the superblock, we do

pub fn vsr_headers(superblock: *const SuperBlockHeader) vsr.Headers.ViewChangeSlice {
    return vsr.Headers.ViewChangeSlice.init(
        if (superblock.vsr_state.log_view < superblock.vsr_state.view)
            .do_view_change
        else
            .start_view,
        superblock.vsr_headers_all[0..superblock.vsr_headers_count],
    );
}

Let's assert that this command-deducing logic is consistent with what we actually write.