feat: Go port of the panel in panel-go/ — drop-in replacement for server.js

Pure-stdlib Go implementation of the whole control plane: same
nexusgate.json schema (unknown fields preserved verbatim, explicit
null/false tri-states modeled), same HTTP API and error strings,
embedded frontend (NG_PUBLIC_DIR override), self-implemented scrypt
auth, X25519 Reality keys, panel-hosted keyStore and heartbeat
reconciliation included. Version stays 0.6.8.

Verified: Go tests 33/33 (scrypt + Reality vectors generated from
the Node implementation, subscription goldens byte-identical),
Node suite 52/52 untouched, differential E2E against the Node
panel 50/50 including two-way DB swap. See panel-go/PARITY.md
and panel-go/INSTALL-GO.md.

panel-go/build.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+# Build the Go panel: refresh the embedded static assets from ../public,
+# then compile. The resulting binary serves the UI from the embedded copy
+# unless NG_PUBLIC_DIR points at a directory on disk.
+set -e
+cd "$(dirname "$0")"
+rm -rf embed_public
+cp -R ../public embed_public
+CGO_ENABLED=0 go build -trimpath -o nexusgate-panel .