Add Nix flake for mcproxyctl

Vendor dependencies and expose mcproxyctl binary via nix build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/modernc.org/libc/mem_expvar.go

@@ -0,0 +1,12 @@
+//go:build libc.memexpvar
+
+package libc
+
+import "expvar"
+
+func init() {
+	// make sure to build with -tags=memory.counters to have the actual data accumulated in memory allocator
+	expvar.Publish("memory.allocator", expvar.Func(func() interface{} {
+		return MemStat()
+	}))
+}