[M5+] add resolver::nixpkgs_probe (nix eval wrapper)

src/resolver/resolver.cppm

@@ -1,5 +1,29 @@
 export module cargoxx.resolver;
 
+import std;
 import cargoxx.util;
 import cargoxx.exec;
 import cargoxx.linkdb;
+
+export namespace cargoxx::resolver {
+
+// What `nix eval nixpkgs#<pkg>` reports for a package: a confirmation that
+// the attribute exists, a best-effort version string, and the realized
+// nix-store path so later probes can scan its installed CMake configs.
+struct NixpkgsInfo {
+    std::string attr;       // the queried name, e.g. "simdjson"
+    std::string version;    // empty when the derivation has no version
+    std::string out_path;   // absolute /nix/store/... path
+};
+
+// Pure parser exposed for unit testing. Accepts the raw JSON returned by
+// `nix eval --json --apply 'p: { ... }'` and extracts NixpkgsInfo.
+auto parse_nix_eval_json(std::string_view attr, std::string_view json)
+    -> util::Result<NixpkgsInfo>;
+
+// Runs `nix eval nixpkgs#<attr> --json --apply ...` via exec::run. Returns
+// `ResolutionUnknownPackage` when the attribute is missing,
+// `ResolutionNetworkError` on timeout or evaluator errors.
+auto nixpkgs_probe(const std::string& attr) -> util::Result<NixpkgsInfo>;
+
+} // namespace cargoxx::resolver