ci: fix test_subagent strcasestr crash; llvm-mingw Windows zip

clang under strict C11 left strcasestr undeclared (returns int → pointer
truncation → SIGSEGV). Parse Content-Length portably; keep -Werror on
make check. Drop Nix for windows-cross (root /nix fails) and use pinned
llvm-mingw UCRT instead; Alpine musl gets linux-headers for audit.h.

tests/test_subagent.c

@@ -18,6 +18,7 @@
 #define CST_TEST_SOCKETS
 #include "cst_test_util.h"
 
+#include <ctype.h>
 #include <curl/curl.h>
 #include <pthread.h>
 #include <stdatomic.h>
@@ -52,6 +53,20 @@ static void send_all(test_sock_t fd, const char *s, size_t n) {
   }
 }
 
+/* Portable Content-Length lookup (avoid GNU strcasestr; undeclared under
+ * strict C11 it is assumed to return int and the pointer truncates). */
+static const char *find_content_length(const char *hdrs) {
+  static const char key[] = "content-length:";
+  for (const char *p = hdrs; *p; p++) {
+    size_t i = 0;
+    while (key[i] && tolower((unsigned char)p[i]) == (unsigned char)key[i])
+      i++;
+    if (key[i] == '\0')
+      return p + (sizeof(key) - 1);
+  }
+  return NULL;
+}
+
 static void sse(test_sock_t fd, cJSON *delta, const char *finish) {
   cJSON *o = cJSON_CreateObject();
   cJSON *ch = cJSON_AddArrayToObject(o, "choices");
@@ -205,7 +220,7 @@ static void handle(test_sock_t fd, const char *body_txt) {
         const char *rep = cJSON_IsString(c) ? strstr(c->valuestring,
                                                      "child report ")
                                             : NULL;
-        if (strcmp(r->valuestring, "tool") == 0 && rep)
+        if (cJSON_IsString(r) && strcmp(r->valuestring, "tool") == 0 && rep)
           cst_buf_printf(&b, " [%.14s]", rep);
       }
       reply_text(fd, b.buf);
@@ -230,8 +245,8 @@ static void *conn_main(void *p) {
       char *e = strstr(req.buf, "\r\n\r\n");
       if (e) {
         hdr_end = (size_t)(e - req.buf) + 4;
-        char *cl = strcasestr(req.buf, "Content-Length:");
-        need = cl ? atol(cl + 15) : 0;
+        const char *cl = find_content_length(req.buf);
+        need = cl ? atol(cl) : 0;
       }
     }
     if (hdr_end && (long)(req.len - hdr_end) >= need)