A memory cap for each Trial #1015

Parent

#977

What to build

• Each Trial and each Grader runs in a scope of its own, from systemd-run --user --scope, with MemoryHigh, MemoryMax and MemorySwapMax. It also raises its own oom_score_adj, so the out-of-memory killer picks it before the factory.
• A Trial killed by its cap is void (Void Trials #1007). An Arm that keeps hitting the cap is reported.
• Defaults: the caps' defaults come from Live checks for the Trial sandbox #1009's measurement, and are documented with the User config's defaults.

It needs lingering (#1011).

Acceptance criteria

• A Trial that goes over a small test cap dies alone and is void, and the others carry on.
• Trials and Graders raise their own oom_score_adj.
• The caps' defaults are documented.