wgengine/netstack: block 4via6 forwards to host-scoped targets (#20866)

The packet filter only sees a via address's outer ULA, so netstack unconditionally dialed whatever IPv4 was embedded in it.

This change refuses TCP, UDP, and ping relays to host-scoped destinations after UnmapVia.

Credit to the Anthropic infrastructure security team for finding and reporting.

Fixes https://github.com/tailscale/corp/issues/46646

Change-Id: I93d8e27a2eddbce7eb3f8c5fa4677f8de3a8ed9e

Signed-off-by: Mike Jensen <mikej@tailscale.com>