fix: separate cargo target-dirs to avoid parallel lock contention, add pam smoke test

tests/pam_smoke_test.c

@@ -0,0 +1,18 @@
+#include <dlfcn.h>
+#include <stdio.h>
+
+int main(int argc, char *argv[]) {
+    if (argc < 2) {
+        fprintf(stderr, "Usage: %s <libahfail_pam.so>\n", argv[0]);
+        return 1;
+    }
+    void *lib = dlopen(argv[1], RTLD_NOW);
+    if (!lib) { fprintf(stderr, "dlopen: %s\n", dlerror()); return 1; }
+    if (!dlsym(lib, "pam_sm_authenticate")) {
+        fprintf(stderr, "pam_sm_authenticate not found\n");
+        dlclose(lib);
+        return 1;
+    }
+    dlclose(lib);
+    return 0;
+}