nix store path-from-hash-part: Constructing Full Store Paths from Hash Parts

1st chunk of `src/nix/path-from-hash-part.md`

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# Examples

* Return the full store path with the given hash part:

  ```console
  # nix store path-from-hash-part --store https://cache.nixos.org/ 0i2jd68mp5g6h2sa5k9c85rb80sn8hi9
  /nix/store/0i2jd68mp5g6h2sa5k9c85rb80sn8hi9-hello-2.10
  ```

# Description

Given the hash part of a store path (that is, the 32 characters
following `/nix/store/`), return the full store path. This is
primarily useful in the implementation of binary caches, where a
request for a `.narinfo` file only supplies the hash part
(e.g. `https://cache.nixos.org/0i2jd68mp5g6h2sa5k9c85rb80sn8hi9.narinfo`).

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Title: nix store path-from-hash-part: Constructing Full Store Paths from Hash Parts

Summary

The `nix store path-from-hash-part` command takes the hash part of a Nix store path (the 32 characters after `/nix/store/`) and returns the complete store path. This is useful in binary cache implementations, where requests for `.narinfo` files only include the hash part.