This takes the `std::filesystem::path` migration from the CLI layer into the core libraries (libutil, libstore, libexpr, libfetchers, libflake), converting function signatures, settings fields, and locals throughout `file-system.hh`, `archive.hh`, `configuration.hh`, and their implementations. `PathSetting` becomes `Setting<std::filesystem::path>` in `local-overlay-store.hh` and `local-store.hh` with `.get()` calls at use sites, and several `writeFile` overloads collapse now that the `Path`-based wrappers in `file-system.hh` are gone.
Co-authored-by: Amaan Qureshi <git@amaanq.com>
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