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/github.com/oschwald/maxminddb-golang/deserializer.go

@@ -0,0 +1,31 @@
+package maxminddb
+
+import "math/big"
+
+// deserializer is an interface for a type that deserializes an MaxMind DB
+// data record to some other type. This exists as an alternative to the
+// standard reflection API.
+//
+// This is fundamentally different than the Unmarshaler interface that
+// several packages provide. A Deserializer will generally create the
+// final struct or value rather than unmarshaling to itself.
+//
+// This interface and the associated unmarshaling code is EXPERIMENTAL!
+// It is not currently covered by any Semantic Versioning guarantees.
+// Use at your own risk.
+type deserializer interface {
+	ShouldSkip(offset uintptr) (bool, error)
+	StartSlice(size uint) error
+	StartMap(size uint) error
+	End() error
+	String(string) error
+	Float64(float64) error
+	Bytes([]byte) error
+	Uint16(uint16) error
+	Uint32(uint32) error
+	Int32(int32) error
+	Uint64(uint64) error
+	Uint128(*big.Int) error
+	Bool(bool) error
+	Float32(float32) error
+}