Let the agent run unprivileged against a prepared TUN interface

Creating a network interface needs CAP_NET_ADMIN, but that is a one-time
setup step rather than something the agent must hold for its whole life.

SystemTunFactory now attaches to an interface that already exists and only
creates one when it does not. A persistent interface created by root and
owned by the user therefore lets the agent run with no privileges and no
capabilities at all. When attaching, nothing is reconfigured, since doing so
would need exactly the privileges we are avoiding.

New `tsunagi tun-setup` prints the three commands to run once as root,
resolving the derived interface name and overlay address for the network.

This also fixes a real gap: the overlay address was passed to the factory
and thrown away, so an interface the agent created had no address and could
never have received anything. The `tun` crate sets addresses through an
IPv4-only ioctl and cannot assign an IPv6 one at all, so the agent now
verifies the address is present via /proc/net/if_inet6 and refuses with the
exact command to run instead of coming up broken. Doing it in-process would
mean speaking netlink, which is not implemented and is recorded as such.

Not verified on this machine: no sudo is available here, so the privileged
setup and the attach path were not executed end to end.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/dataplane/wireguard/mod.rs

@@ -64,4 +64,4 @@ pub use store::WgKeyStore;
 pub use tun::{MemoryTun, MemoryTunFactory, TunDevice, TunFactory, TunRequest};
 
 #[cfg(feature = "tun-device")]
-pub use tun::SystemTunFactory;
+pub use tun::{SystemTunFactory, interface_exists, interface_has_address, setup_commands};