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/memory/README.md

@@ -0,0 +1,13 @@
+![logo-png](logo.png)
+
+Package memory implements a memory allocator.
+
+## Build status
+
+available at https://modern-c.appspot.com/-/builder/?importpath=modernc.org%2fmemory
+
+Installation
+
+    $ go get modernc.org/memory
+
+[![Go Reference](https://pkg.go.dev/badge/modernc.org/memory.0.svg)](https://pkg.go.dev/modernc.org/memory)