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>

Cargo.lock

@@ -198,6 +198,7 @@ dependencies = [
  "aura-tunnel",
  "clap",
  "ipnetwork",
+ "nix 0.29.0",
  "rustls-pki-types",
  "serde",
  "serde_json",
@@ -1659,6 +1660,18 @@ dependencies = [
  "zeroize",
 ]
 
+[[package]]
+name = "nix"
+version = "0.29.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "71e2746dc3a24dd78b3cfcb7be93368c6de9963d30f43a6a73998a9cf4b17b46"
+dependencies = [
+ "bitflags",
+ "cfg-if",
+ "cfg_aliases",
+ "libc",
+]
+
 [[package]]
 name = "nix"
 version = "0.31.3"
@@ -2799,7 +2812,7 @@ dependencies = [
  "ipnet",
  "libc",
  "log",
- "nix",
+ "nix 0.31.3",
  "thiserror 2.0.18",
  "tokio",
  "tokio-util",