feat: add index-aware recipe resolution with repo support

- fetch_recipe_from_repos() iterates repos by priority, channels, with
  mirror fallback — caches indexes at /cache/indexes/

- Conditional HTTP downloads (curl -z) to only re-fetch stale indexes

- build_registry and fetch-package prefer repos { } over remotes = [...]
  when both are present

- Fallback chain: local .kap → repos (index-aware) → remotes (legacy)

include/kappa/fetch/recipe.hpp

@@ -1,5 +1,7 @@
 #pragma once
 
+#include "kappa/dsl/system.hpp"
+
 #include <string>
 #include <vector>
 
@@ -13,7 +15,12 @@ struct RecipeResult {
     std::string error;
 };
 
+// Legacy: flat URL list (backward compat with remotes = [...])
 RecipeResult fetch_recipe(const std::string& name,
                           const std::vector<std::string>& remotes);
 
+// Index-aware: named repos with channels, mirrors, and cached indexes
+RecipeResult fetch_recipe_from_repos(const std::string& name,
+                                     const std::vector<dsl::RepoDef>& repos);
+
 } // namespace kappa::fetch