docs(task): done-guard-window — done 가드 오탐의 구조적 원인 (조사·설계, 구현 미착수) - #46

ℹ spawn floor 26.6ms (25.4, 26.1, 26.6, 27.1, 29.9); equal-work baseline 36.6ms (38.4, 36.4, 36.6, 38.1, 35.9); cold 63.8ms = 1.74x baseline (62.8, 65.2, 66.3, 60.3, 63.8); checkpoint 65.8ms = 1.80x baseline (64.9, 68.4, 65.8, 68.4, 65.4)