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/AUTHORS

@@ -0,0 +1,13 @@
+# This file lists authors for copyright purposes.  This file is distinct from
+# the CONTRIBUTORS files.  See the latter for an explanation.
+#
+# Names should be added to this file as:
+#     Name or Organization <email address>
+#
+# The email address is not required for organizations.
+#
+# Please keep the list sorted.
+
+CZ.NIC z.s.p.o. <kontakt@nic.cz>
+Edward Betts <edward@4angle.com>
+Jan Mercl <0xjnml@gmail.com>