Boot-time ktr_buf/ktr_entries_mask publication relies on TSO (no explicit read-side barrier, x86-only safe)

Summary

ktr_sysinit (kern_ktr.c:233-242): publishes per-CPU ktr_buf BEFORE updating global ktr_entries_mask BOOT0->full (:241-242). No explicit read-side barrier in ktr_begin_write_entry (:511). On x86 TSO safe (stores observed in program order, cpu_sfence :240 redundant). NOT enforced for weak-order archs (no READ_ONCE/acquire). One-time SI_BOOT2_KLD before userspace; index mask still clamps valid slot on x86. Not exploitable as shipped. Fix: release/acquire accessors or per-CPU mask.