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

src/q/upsert-blob.sql

@@ -0,0 +1,4 @@
+INSERT INTO sidx_blob(blake3, n_bytes)
+VALUES
+(?, ?)
+ON CONFLICT DO NOTHING;