[M5+] add resolver::nixpkgs_git_resolve fallback

src/resolver/resolver.cppm

@@ -158,4 +158,20 @@ auto parse_devbox_resolve(std::string_view json)
 auto devbox_resolve(const std::string& name, const std::string& version)
     -> util::Result<DevboxResolution>;
 
+// Pure: parse the output of
+//   git log --all -S 'version = "<v>"' --pretty='%H %ct' -- pkgs/
+// (one commit per line: "<40-char-sha> <unix-epoch-seconds>") and
+// return the youngest matching SHA, or nullopt on empty input.
+auto pick_youngest_commit(std::string_view git_log_output)
+    -> std::optional<std::string>;
+
+// Searches a local nixpkgs clone for the youngest commit that
+// introduced `version = "<version>"` under pkgs/. `repo_path`
+// defaults to ~/.cache/cargoxx/nixpkgs/ — when the directory doesn't
+// exist the repo is cloned lazily (multi-GB, only on first miss).
+// 404-equivalent: ResolutionVersionNotFound.
+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>;
+
 } // namespace cargoxx::resolver