truncated names: perhaps assumes a fixed store prefix length?

I tried to diff some derivations in `/nix/store/` to some derivations in `/tmp`, and got truncated names:

```
- 4rjz3965j8qjaf5qnfii1991cyrbrqxr-nixos-graphical-25.05pre793008.f02fddb8acef-x86_64-linux.iso.drv:{out}
+ /nix/store/vbs21r5zn550hhsmarl22xz68sbz12lv-nixos-graphical-25.05pre793008.f02fddb8acef-x86_64-linux.iso.drv:{out}
• The set of input derivation names do not match:
- age_eltorito
- e-info
- rectory
- ux.cfg
+ efi-directory
+ efi-image_eltorito
+ isolinux.cfg
• The set of input derivations named `closure-info` do not match
+ /nix/store/46psyizz5a7ny5cm70b33l81lb00v2sv-closure-info.drv:{out}
• The environments do not match:
```

I suspect something assumes the store prefix would have a fixed length across all derivations

Reproduce the reported diff between derivations in /nix/store/ and /tmp using the example paths and input names in the issue body. Search the nix-diff code for fixed-length assumptions when formatting or comparing store paths, then confirm that full derivation and input names are preserved in the output.

Written by the indexing model from the issue text.