One Semaphore(2) gated every compile, so while two ESP32 builds ran (cmake +
ninja, minutes on a cold cache) an Arduino Uno blink sat 'compiling' for as
long as they took. Two lanes now: heavy (ESP-IDF, still 2 — the VPS load
constraint is real) and light (arduino-cli boards, 3), plus a console line
when a job is queued so the wait is visible instead of looking like a hung
build. The per-target lock that keeps concurrent builds out of one ESP-IDF
build dir is unchanged.
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