test: require diagnostic on reject rows, close crash-vacuity (#20)

Reject helpers checked only rc!=0, so a SEGFAULT (exit 139, or the
driver's "w6c failed" exit 1) counted as a clean reject -- a wwstage
crash could pass vacuously (it did, latently, on bodied bare-... pre
#11). Every reject row now captures stderr and requires the actual
diagnostic substring (rc!=0 AND strstr) across all 16 reject tests; a
crash emits no diagnostic, so it now fails. This is the
differential-reject backstop (#15): both stages must cleanly reject
with the expected message.

Two genuine cstage/wwstage diagnostic-body divergences are documented
inline via per-stage substrings, not papered over (845 tuple parse,
catA_f2 tuple arity); catalogued in #21.

test/wcc/742_parse_error.c

@@ -12,6 +12,7 @@
  */
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <unistd.h>
 #include <sys/wait.h>
 
@@ -34,17 +35,41 @@ write_file(const char *path, const char *content)
 	return 0;
 }
 
+static int
+filehas(const char *path, const char *needle)
+{
+	char buf[8192];
+	FILE *f = fopen(path, "rb");
+	if (!f) return 0;
+	size_t n = fread(buf, 1, sizeof buf - 1, f);
+	fclose(f);
+	buf[n] = '\0';
+	return strstr(buf, needle) != NULL;
+}
+
+/* check_fail — the tool must exit non-zero AND emit a parse diagnostic.
+ * Both stages report "expected identifier" first on this fixture; a crash
+ * exits non-zero with no diagnostic, so the substring distinguishes a
+ * clean reject from a segfault (#20). */
 static int
 check_fail(const char *tool, const char *src)
 {
-	char cmd[4096];
-	snprintf(cmd, sizeof cmd, "%s %s >/dev/null 2>/dev/null", tool, src);
+	char cmd[4096], errf[64];
+	snprintf(errf, sizeof errf, "/tmp/parse_err_e_%d.txt", getpid());
+	snprintf(cmd, sizeof cmd, "%s %s >/dev/null 2>%s", tool, src, errf);
 	int rc = runwait(cmd);
+	int hasmsg = filehas(errf, "expected identifier");
+	unlink(errf);
 	if (rc == 0) {
 		fprintf(stderr, "742 FAIL: %s exited 0 on parse-error fixture\n",
 		    tool);
 		return -1;
 	}
+	if (!hasmsg) {
+		fprintf(stderr, "742 FAIL: %s nonzero exit but missing parse "
+		    "diagnostic (a crash, not a clean reject)\n", tool);
+		return -1;
+	}
 	return 0;
 }