fix(perf): boundary perf 예산을 동일 작업량 baseline 대비 비율로 교체 (CI flake) - #44

# spawn floor 22.1ms (20.8, 22.1, 21.8, 22.2, 22.1); equal-work baseline 35.8ms (34.8, 35.8, 37.6, 37.9, 34.8); cold 62.5ms = 1.75x baseline (62.0, 61.5, 63.2, 62.5, 63.7); checkpoint 67.5ms = 1.89x baseline (66.1, 68.4, 67.5, 65.3, 67.6)