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>

CMakeLists.txt

@@ -32,5 +32,6 @@ add_executable(kappa
   src/tools/format.cpp
   src/tools/doctor.cpp
   src/resolve/plan.cpp
+  src/fetch/fetch.cpp
 )
 target_include_directories(kappa PRIVATE include)