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/mathutil/sqr.go

@@ -0,0 +1,12 @@
+//go:build !riscv64 && !loong64
+// +build !riscv64,!loong64
+
+package mathutil
+
+import "github.com/remyoudompheng/bigfft"
+
+func (f *float) sqr() {
+	f.n = bigfft.Mul(f.n, f.n)
+	f.fracBits *= 2
+	f.normalize()
+}