Add Nix flake for mcrctl

Vendor dependencies and expose mcrctl binary via nix build.

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

vendor/github.com/remyoudompheng/bigfft/arith_decl.go

@@ -0,0 +1,33 @@
+// Copyright 2010 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bigfft
+
+import (
+	"math/big"
+	_ "unsafe"
+)
+
+type Word = big.Word
+
+//go:linkname addVV math/big.addVV
+func addVV(z, x, y []Word) (c Word)
+
+//go:linkname subVV math/big.subVV
+func subVV(z, x, y []Word) (c Word)
+
+//go:linkname addVW math/big.addVW
+func addVW(z, x []Word, y Word) (c Word)
+
+//go:linkname subVW math/big.subVW
+func subVW(z, x []Word, y Word) (c Word)
+
+//go:linkname shlVU math/big.shlVU
+func shlVU(z, x []Word, s uint) (c Word)
+
+//go:linkname mulAddVWW math/big.mulAddVWW
+func mulAddVWW(z, x []Word, y, r Word) (c Word)
+
+//go:linkname addMulVVW math/big.addMulVVW
+func addMulVVW(z, x []Word, y Word) (c Word)