Per design: a hot CPU is a global hardware event, so every engine pauses — not just
the CPU-heat source. Reverts the per-engine CPU exemption from 0.1.75 (front
_thermal_loop) and 0.1.76 (engine-level wait_until_safe): the shared-CPU term again
applies to all engines. The original "radeon stuck while colibri ran free" bug is
addressed the right way — by making colibri ACTUALLY honour the pause (targeted
SIGSTOP that keeps the engine's HTTP alive, and colibri's cooperative PAUSE/RESUME
mux frame), both retained — so colibri stops, the CPU cools, and all engines resume
together instead of the innocent engine being stranded.

Removed the now-unused _scan_pgroup_cpu/_engine_cpu_relevant helpers and
_self_cpu_relevant.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LoSpEthysqmseCc6Geizty