Investigation: macOS arm64 grew 1MB (43→44MB) but slope was 1123
KB/hr. Linux amd64 grew 3MB (25→28MB) but slope was only 424 KB/hr.
The linear regression fits noise on short runs — small timing
fluctuations produce wildly different slopes on nearly flat data.
For 10-min quick soak: use RSS ceiling (200MB) + ratio (3.0x) +
FD drift + idle CPU. Slope is still reported but not enforced.
For 30+ min runs: slope enforcement at 500 KB/hr (enough samples
for reliable regression).