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>

examples/foo.kap

@@ -5,6 +5,7 @@
 package "foo" {
     const version = "1.2.3"
     const source  = "https://example.com/foo-${version}.tar.gz"
+    sha256 = "e127a709cba24c76de8936cb7083dd768f28cd37eb010492e2f19b71eb1294e4"
     license = "MIT"
 
     provides = ["libfoo.so.1", "foo"]