Fragile zeroing contract for fill_kinfo_lwp aggregation (+=)

Summary

fill_kinfo_lwp does NOT bzero output (unlike fill_kinfo_proc:126 and fill_kinfo_proc_kthread:286). Uses += on tick/pctcpu/slptime fields(:257-262) and ruadd(:267). Correctness depends on caller pre-zeroing. Current kernel caller correct (kern_proc.c:1601,1613). Latent regression risk.