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/CONTRIBUTORS

@@ -0,0 +1,15 @@
+# This file lists people who contributed code to this repository.  The AUTHORS
+# file lists the copyright holders; this file lists people.
+#
+# Names should be added to this file like so:
+#     Name <email address>
+#
+# Please keep the list sorted.
+
+Bodecker DellaMaria <bojdell@gmail.com>
+Edward Betts <edward@4angle.com>
+Faiz Abbasi <faizamodo@gmail.com>
+Gary Burd <gary@beagledreams.com>
+Jan Mercl <0xjnml@gmail.com>
+Muhammad Surya <surya.asriadie@gmail.com>
+Santiago De la Cruz <delacruzsantiago12@gmail.com>