[M2] add curated linkdb + semver matcher

src/linkdb/linkdb.cppm

@@ -1,3 +1,56 @@
 export module cargoxx.linkdb;
 
+import std;
 import cargoxx.util;
+
+export namespace cargoxx::linkdb {
+
+struct Recipe {
+    std::string nixpkgs_attr;
+    std::string find_package; // post-substitution
+    std::vector<std::string> targets; // post-substitution
+    std::string source; // 'curated' | 'manual' | etc.
+
+    bool operator==(const Recipe&) const = default;
+};
+
+} // namespace cargoxx::linkdb
+
+namespace cargoxx::linkdb::detail {
+
+struct CuratedRecipe {
+    std::string version_range;
+    std::string nixpkgs_attr;
+    std::string find_package;
+    std::vector<std::string> targets;
+    bool components_supported = false;
+};
+
+} // namespace cargoxx::linkdb::detail
+
+export namespace cargoxx::linkdb {
+
+class Database {
+  public:
+    static auto open() -> util::Result<Database>;
+
+    auto resolve(const std::string& package, const std::string& version,
+                 const std::vector<std::string>& components = {})
+        -> util::Result<Recipe>;
+
+    auto add_manual(const std::string& package, const std::string& version_range,
+                    const Recipe& r) -> util::Result<void>;
+
+  private:
+    Database() = default;
+    std::map<std::string, std::vector<detail::CuratedRecipe>> curated_;
+};
+
+// Pure helpers exported for unit testing.
+auto substitute_components(std::string find_package, const std::vector<std::string>& components)
+    -> std::string;
+
+auto expand_targets(const std::vector<std::string>& templates,
+                    const std::vector<std::string>& components) -> std::vector<std::string>;
+
+} // namespace cargoxx::linkdb