feat(cli): auto-NAT + privilege drop + Windows named-pipe admin

Three v2-hardening features in aura-cli, one pass:

- nat::NatGuard: RAII auto-config of IP forwarding + MASQUERADE on server
  startup. Linux (sysctl ip_forward + iptables -t nat MASQUERADE) and
  macOS (sysctl ip.forwarding + pfctl with /tmp/aura-nat.conf). dry_run
  works on every platform (logs "would run: ..."). Reverts everything in
  Drop. New [server.nat] {auto, egress_iface, dry_run}; absent section =
  back-compat no-op. Removes v1's "manual NAT/forwarding" step.
- privdrop::drop_to_user: drop euid/gid after binding TUN + privileged
  ports. Linux setresuid/setresgid, macOS setgid+setuid (permanent drop),
  Windows no-op with warning. New [server] / [client] run_as = "..."
  (optional). Skipped with info-log if already non-root.
- admin: split transport into cfg(unix) Unix-socket and cfg(windows) Tokio
  named-pipe modules sharing one JSON-line serve/request flow over
  AsyncRead/AsyncWrite. DEFAULT_SOCKET = "/tmp/aura-admin.sock" on Unix,
  r"\\.\pipe\aura-admin" on Windows. Removes v1's "admin Unix-only".

Deps: nix 0.29 user feature under [target.'cfg(unix)'.dependencies] (cli-
local, not workspace). Workspace: 155 tests passed (+13), clippy -D warnings
clean, fmt clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

config/client.toml.example

@@ -9,6 +9,12 @@ server_addr = "203.0.113.10:443"
 # Outer-TLS SNI (camouflage hostname) presented to the server. Also the name verified
 # inside the Aura handshake against the server certificate's SAN.
 sni = "cdn.example.com"
+# Optional: drop privileges to this non-root user AFTER the TUN device has been brought up.
+# Recommended when `aura client` is launched via sudo so the long-running router loop runs
+# as an ordinary user. Linux uses setresuid/setresgid; macOS uses setgid/setuid; Windows is a
+# no-op (use a service account instead). When omitted (or already running as non-root) no
+# privilege change happens.
+# run_as = "nobody"
 
 [pki]
 # Trust anchor (the Aura CA) and this client's leaf cert/key, all PEM.