fix: fetcher security — fork+execve, hash verification, tar safety, CLI --help

B1: Replaced all std::system()/popen() with fork+execve — zero shell injection
B2+B3: Added sha256/sha512/md5 fields to PackageDef, parser support, actual hash comparison in verify_hash()
B4: tar --no-same-owner --no-same-permissions, zip uses unzip instead of tar
B5: Patch sha256 verified before application via -i flag (no shell redirect)
B6: kappa fetch --help now prints usage and exits 0
Also: removed dead ternary code, added <cstdlib>/<sys/wait.h>/<unistd.h>

include/kappa/dsl/ast.hpp

@@ -63,6 +63,9 @@ struct PackageDef {
     std::string name;
     std::string version;
     std::string source;
+    std::string sha256;
+    std::string sha512;
+    std::string md5;
     std::string license;
     std::vector<Dependency> depends;
     std::vector<std::string> provides;