[RT-E9] An owned seam between generated and hand-written code. Lands paired with GEN-E5 #488

Part of #350, the quality-refactoring roadmap (road to 1.0, step 3).

Goal: An owned seam between generated and hand-written code. Lands paired with GEN-E5.

Lane: Runtime + Generator. Status: todo.

Every PR of this epic starts its title with [RT-E9]. The finding issues it closes are linked as sub-issues; this issue closes when the epic's last PR merges and master CI is green.