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

src/linkdb/linkdb.cppm

@@ -131,6 +131,12 @@ class Database {
     std::unique_ptr<detail::OverlayState> overlay_;
 };
 
+// Returns the default overlay-database path:
+//   $XDG_CACHE_HOME/cargoxx/linkdb.sqlite   (if XDG_CACHE_HOME is set)
+//   $HOME/.cache/cargoxx/linkdb.sqlite       (else if HOME is set)
+//   <cwd>/.cargoxx-linkdb.sqlite             (final fallback)
+auto default_overlay_path() -> std::filesystem::path;
+
 // Pure helpers exported for unit testing.
 auto substitute_components(std::string find_package, const std::vector<std::string>& components)
     -> std::string;