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/modernc.org/mathutil/sqr_std.go

@@ -0,0 +1,10 @@
+//go:build riscv64 || loong64
+// +build riscv64 loong64
+
+package mathutil
+
+func (f *float) sqr() {
+	f.n.Mul(f.n, f.n)
+	f.fracBits *= 2
+	f.normalize()
+}