[M6] resolver: pkg-config probe + codegen for PkgConfig

src/linkdb/linkdb.cppm

@@ -15,6 +15,16 @@ struct Recipe {
     std::vector<std::string> targets; // post-substitution
     std::string source; // 'curated' | 'manual' | etc.
 
+    // When set, the dep is consumed via PkgConfig instead of a normal
+    // find_package. Codegen emits
+    //   find_package(PkgConfig REQUIRED)
+    //   pkg_check_modules(<UPPER> REQUIRED IMPORTED_TARGET <pkg_config_module>)
+    // and the recipe's `targets` list holds the synthesized
+    // PkgConfig::<UPPER> entries. `find_package` for these recipes is
+    // the literal string "PkgConfig REQUIRED" — kept consistent so
+    // overlay rows don't need a sentinel.
+    std::optional<std::string> pkg_config_module;
+
     bool operator==(const Recipe&) const = default;
 };
 
@@ -29,6 +39,7 @@ struct OverlayRow {
     std::vector<std::string> targets;
     std::string source;
     std::int64_t verified_at = 0;
+    std::optional<std::string> pkg_config_module;
 };
 
 // RAII wrapper for an open sqlite3 connection used by the overlay database.