zod: modernize package expression and fix pname scoping
- switch `sha256` to `hash` with SRI-style value
- replace interpolated `${pname}` in sub-derivations with explicit names:
- zod-engine-engine
- zod-engine-map_editor
- zod-engine-launcher
- zod-engine-assets
- drop unnecessary `with lib;` in `meta` and fully qualify attributes
(`lib.maintainers`, `lib.licenses`, `lib.platforms`)
This improves attribute hygiene, avoids accidental pname shadowing,
and aligns the package with current Nixpkgs conventions.