Repeated builds were getting SIGTERM'd or thrashing into swap when
competing with other heavy concurrent builds on the same machine.
Wraps the build in `systemd-run --user --scope` with a low CPU/IO
weight and a soft memory cap (throttles rather than kills), and caps
GOMAXPROCS so cgo doesn't spawn as many parallel cc1plus processes
compiling the vendored Qt6 binding layer.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
