Tailscale HEAD go.mod now requires go >= 1.26.5; build images pinned
1.26.4. Bump go.mod, the four Go Dockerfiles, and the nixpkgs pin
(staging-next-26.05 ships go_1_26 1.26.5; unstable still lags).
A raw coder/websocket dial and the real tailscale.com js/wasm control client under Node, both against headscale alongside normal Tailscale clients.
Updates #3357