[RT-E6] One child-process core for hosting, with one owner for platform checks. Characterization tests merged first #485

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

Goal: One child-process core for hosting, with one owner for platform checks. Characterization tests merged first.

Lane: Runtime. Status: active. Merged so far: #407, #408, #461.

Every PR of this epic starts its title with [RT-E6]. 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.