How does PostgreSQL physically order new records on disk (after a cluster on primary key)?
Rows in postgresql have no fixed order. Not only are records placed where ever there is free space records can also move around. This is because when a row is updated a new version of the row is created in a new location while the old version continues to live in it's old location until it's removed by vacuum.

A CLUSTER operations sorts all rows but does indeed not affect how postgresql add's rows. So the data won't stay sorted. However postgresql keeps statistics among which is the correlation for each column between the order of the rows in the tables and the sorted order of that column. So the planner can still optimize it's plan based on the statistics that tell it the table is still mostly sorted even if some rows have been added after a cluster operation (or have been moved by updates).