[M5+] realize candidate output before nix-cmake-scan

src/resolver/resolver.cppm

@@ -33,6 +33,18 @@ auto parse_nix_eval_json(std::string_view attr, std::string_view json)
 // `ResolutionNetworkError` on timeout or evaluator errors.
 auto nixpkgs_probe(const std::string& attr) -> util::Result<NixpkgsInfo>;
 
+// Materializes a flake attribute on disk by shelling out to
+//   nix build --no-link --print-out-paths nixpkgs#<flake_attr>
+// and returns the absolute store path. The eval-only `nixpkgs_probe`
+// computes the path the output *would* have, but it isn't actually
+// present on disk until it's been built / fetched from the binary
+// cache; nix_cmake_scan needs it on disk to walk lib/cmake. Use
+// `realize_path("libclang.dev")` to scan a dev output, etc.
+//
+// `ResolutionUnknownPackage` for unknown attrs, `ResolutionNetworkError`
+// for build / network errors.
+auto realize_path(const std::string& flake_attr) -> util::Result<std::string>;
+
 // One CMake config-file's IMPORTED targets together with the find_package
 // expression derived from its filename stem.
 struct NixCmakeCandidate {