[M5+] use devbox attr_paths for pinned-dep nixpkgs attr

src/resolver/resolver.cppm

@@ -174,15 +174,25 @@ auto nixpkgs_git_resolve(const std::string& name, const std::string& version,
                          std::optional<std::filesystem::path> repo_path = std::nullopt)
     -> util::Result<std::string>;
 
+// What `resolve_version` returns: a nixpkgs commit and the attribute
+// path under which the package lives at that commit. The attr is
+// authoritative *for that rev* — the cargoxx-curated linkdb attrs
+// (e.g. `fmt_10`) only apply to the unpinned `nixos-unstable` set.
+// When devbox supplies multiple attr paths the first is canonical.
+struct ResolvedVersion {
+    std::string nixpkgs_rev;
+    std::string nixpkgs_attr;
+};
+
 // Top-level orchestrator: try `devbox_resolve` first, then
-// `nixpkgs_git_resolve` as fallback. Returns a 40-char nixpkgs SHA, or
+// `nixpkgs_git_resolve` as fallback. Returns the rev + attr or
 // `ResolutionVersionNotFound` when both probes come back empty.
 //
-// Use this from `cargoxx add <pkg>@<concrete-ver>` to capture the rev
-// that lockfile/codegen will pin. Wildcards (`*`, empty) should NOT be
-// passed — they are not concrete versions and the resolver returns
-// `ResolutionVersionNotFound` for them by design.
+// Use this from `cargoxx add <pkg>@<concrete-ver>` to capture the pin
+// that lockfile/codegen will use. Wildcards (`*`, empty) should NOT
+// be passed — they are not concrete versions and the resolver
+// returns `ResolutionVersionNotFound` for them by design.
 auto resolve_version(const std::string& name, const std::string& version)
-    -> util::Result<std::string>;
+    -> util::Result<ResolvedVersion>;
 
 } // namespace cargoxx::resolver