test: add end-to-end integration across dash/bash/zsh

tests/integration/fixture/main.c

@@ -0,0 +1,63 @@
+/*
+ * main.c - integration-fixture entry point (plan todo 26).
+ *
+ * A REAL C23 program exercising both fixture features end to end:
+ *   - pthread: pthread_create()/pthread_join() run a worker thread (guarded
+ *     by the HAVE_PTHREAD define the generated config.h wrote), printing a
+ *     grep-able marker the runner asserts on;
+ *   - math: sin(0.5) from libm, printed with a fixed format the runner
+ *     greps (the link would fail without -lm, so a successful run proves
+ *     the LIBS accumulation reached the Makefile).
+ *
+ * _POSIX_C_SOURCE must be defined BEFORE the first include: under strict
+ * -std=c23 glibc sets __STRICT_ANSI__ and hides pthread_create's
+ * declaration, which C23 turns into an implicit-declaration ERROR
+ * (the same trap recorded for src/ in the project learnings).
+ */
+
+#ifndef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200809L
+#endif
+
+#include "config.h"
+#include "demo.h"
+
+#include <math.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#ifdef HAVE_PTHREAD
+#include <pthread.h>
+
+static void *
+worker(void *arg)
+{
+    (void)arg;
+    printf("worker thread ran\n");
+    return NULL;
+}
+#endif
+
+int
+main(void)
+{
+    double s = sin(0.5);
+
+    printf("hellothreads demo: sin(0.5) = %.4f\n", s);
+    printf("demo_compute(2) = %d\n", demo_compute(2));
+#ifdef HAVE_PTHREAD
+    {
+        pthread_t t;
+
+        if (pthread_create(&t, NULL, worker, NULL) != 0) {
+            fprintf(stderr, "pthread_create failed\n");
+            return EXIT_FAILURE;
+        }
+        if (pthread_join(t, NULL) != 0) {
+            fprintf(stderr, "pthread_join failed\n");
+            return EXIT_FAILURE;
+        }
+    }
+#endif
+    return EXIT_SUCCESS;
+}