Reports whether a node's online/offline transition forces peers to recompute their netmap. True for subnet routers, relay targets (tailscale.com/cap/relay), and via targets; false otherwise.
The relay-target IP set and via-target tag set are precompiled from the grants in updateLocked, alongside the existing filter, so the per-node check is a cheap set lookup. Keyed on the node itself, so an ordinary node in a tailnet that uses relay or via for other nodes is still classified as not needing a recompute.
Updates #3293
bceac495f9