init

First attempt at both Rust and snix-castore.
I'll start by putting together a single-file CLI app,
no splitting modules out until needed, no separation of concerns.

Currently, just the fetching is implemented:

```
$ sidx pkgs/dev*/cuda-*/cuda/manifests/*.json
```

npins: init

npins/sources.json

@@ -0,0 +1,11 @@
+{
+  "pins": {
+    "nixpkgs": {
+      "type": "Channel",
+      "name": "nixpkgs-unstable",
+      "url": "https://releases.nixos.org/nixpkgs/nixpkgs-25.05pre782598.18dd725c2960/nixexprs.tar.xz",
+      "hash": "1p7kgyph7xkj57p19nbxpycmbchc6d9gwdznsmxhymrzyzi3if21"
+    }
+  },
+  "version": 3
+}