Object lifetime follows every holder, not just the creator: ntsync_retain and
ntsync_release let a process that receives a relayed handle keep the object
alive, which is what a duplicated fd does under the kernel driver. The shm
layout version is now 10, so a v9 region file is rejected with -ESTALE.

The spin tail (NTSYNC_SPIN_TAIL_US) is bounded by
NTSYNC_SPIN_TAIL_BUDGET_PCT, so a mistuned tail cannot pin a core.