[M8] reusable libraries: install layout + cargoxx-path deps

src/manifest/manifest.cppm

@@ -5,10 +5,17 @@ import cargoxx.util;
 
 export namespace cargoxx::manifest {
 
+enum class DepSource {
+    Auto,         // string form or { version = ... } only → existing resolver chain
+    CargoxxPath,  // { path = "../foo" } → recursive cargoxx build
+};
+
 struct Dependency {
     std::string name;
     std::string version_spec;
     std::vector<std::string> components;
+    DepSource source = DepSource::Auto;
+    std::optional<std::string> path;  // when source == CargoxxPath
 
     bool operator==(const Dependency&) const = default;
 };