[M5+] wire resolver::discover into cargoxx add

src/resolver/resolver.cppm

@@ -115,4 +115,25 @@ struct VerifyLinkRequest {
 auto verify_link(const VerifyLinkRequest& req, const BuildFn& build_fn)
     -> util::Result<void>;
 
+// What discover() returns: the verified Recipe and a tag identifying
+// which probe yielded it ("conan", "vcpkg", or "nix-probe").
+struct Discovered {
+    linkdb::Recipe recipe;
+    std::string source;
+};
+
+// Walks the full auto-resolution chain for a package not present in the
+// curated linkdb or the user's overlay:
+//   1. nixpkgs_probe(name) — confirms the attribute exists, captures
+//      version + out_path
+//   2. for each of conan_probe, vcpkg_probe, nix_cmake_scan(out_path,…):
+//      build a candidate linkdb::Recipe, run verify_link on it, return
+//      on first success
+//   3. all candidates failed → ResolutionUnsatisfiable
+auto discover(const std::string& name, const std::string& version_spec,
+              const std::vector<std::string>& components,
+              const std::filesystem::path& overlay_path,
+              const std::filesystem::path& scratch_root, const BuildFn& build_fn)
+    -> util::Result<Discovered>;
+
 } // namespace cargoxx::resolver