Add Nym mixnet transport and single-container demo

Add an outbound-only Nym mixnet transport that tunnels FMP peer links
through a local nym-socks5-client SOCKS5 proxy into the Nym mixnet. It
structurally mirrors the Tor SOCKS5 transport (connection pool,
connect-on-send background promotion, FMP-v0 framing reused from TCP)
with the onion, inbound-listener, and control-port machinery removed.

Wires the transport through the full TransportHandle dispatch, NymConfig
(standard transport-instance pattern), and node instantiation, and
surfaces its counters in fipstop. Includes a mock SOCKS5 harness and unit
coverage for the address-parsing paths.

Also adds an isolated single-container example
(examples/sidecar-nostr-mixnet-relay/) demonstrating FIPS peering across
the mixnet end to end. No new crate dependencies: tokio_socks, socket2,
and futures are already pulled in by the Tor transport.

src/bin/fipstop/ui/transports.rs

@@ -481,6 +481,26 @@ fn draw_transport_detail(frame: &mut Frame, app: &App, area: Rect, t: &serde_jso
                     &helpers::nested_u64(t, "stats", "connect_refused"),
                 ));
             }
+            "nym" => {
+                lines.push(helpers::kv_line(
+                    "MTU Exceeded",
+                    &helpers::nested_u64(t, "stats", "mtu_exceeded"),
+                ));
+                lines.push(helpers::kv_line(
+                    "SOCKS5 Errors",
+                    &helpers::nested_u64(t, "stats", "socks5_errors"),
+                ));
+                lines.push(Line::from(""));
+                lines.push(helpers::section_header("Connections"));
+                lines.push(helpers::kv_line(
+                    "Established",
+                    &helpers::nested_u64(t, "stats", "connections_established"),
+                ));
+                lines.push(helpers::kv_line(
+                    "Timeouts",
+                    &helpers::nested_u64(t, "stats", "connect_timeouts"),
+                ));
+            }
             "ethernet" => {
                 lines.push(Line::from(""));
                 lines.push(helpers::section_header("Beacons"));