fix(test): make test_stops build on Windows (mkdtemp compat shim)

test_stops.c uses POSIX mkdtemp() to make a scratch dir in the CWD, but
MinGW-w64 does not declare mkdtemp, so the test failed to compile on the
Windows job - and only there:

  tests/test_stops.c:73:9: error: implicit declaration of function 'mkdtemp';
     did you mean 'mktemp'? [-Wimplicit-function-declaration]
  make: *** [Makefile:318: tests/test_stops.exe] Error 1

That halted `make test` at the C-test stage on Windows (test_uring is
correctly Linux-only, so test_stops was the only blocker).

Added a compat_mkdtemp shim to compat.h, following the file's existing
convention (every platform difference lives there; the .c stays clean):
_mktemp fills the trailing X's in place (same contract as mkdtemp), then
_mkdir creates the directory. Also added <direct.h> for _mkdir. On Linux
compat.h is a complete no-op, so POSIX mkdtemp is untouched there.

Verified: test_stops builds clean on MinGW and all 5 sub-cases pass:
  tokenizer special-flag parsing, config/generation_config eos union,
  no-generation_config fallback, both-configs-mutilated tokenizer sweep,
  and the T=NULL validation path.

c/compat.h

@@ -80,6 +80,7 @@ static inline int compat_open_direct(const char *path){
 #endif
 #include <windows.h>
 #include <io.h>
+#include <direct.h>   /* _mkdir (for the mkdtemp shim below) */
 #include <process.h>
 #include <malloc.h>
 #include <fcntl.h>
@@ -304,6 +305,19 @@ static inline int compat_setenv(const char *name, const char *value, int overwri
 }
 #define setenv(name,value,overwrite) compat_setenv(name,value,overwrite)
 
+/* --- mkdtemp -> _mktemp + _mkdir (POSIX mkdtemp assente su Windows) ---
+ * Test binaries (test_stops.c) create a scratch dir in the CWD via a
+ * "name_XXXXXX" template; POSIX mkdtemp fills the X's and mkdirs 0700. The
+ * Windows CRT has _mktemp (in-place, same XXXXXX contract) so we compose it.
+ * Returns the template pointer on success, NULL on failure — matching POSIX. */
+static inline char *compat_mkdtemp(char *tmpl){
+    if(!tmpl) return NULL;
+    if(!_mktemp(tmpl)) return NULL;       /* fills the trailing X's in place */
+    if(_mkdir(tmpl) != 0) return NULL;    /* EEXIST is impossible post-_mktemp */
+    return tmpl;
+}
+#define mkdtemp(tmpl) compat_mkdtemp(tmpl)
+
 #endif /* _WIN32 */
 
 /* --- compat_aligned_free su piattaforme diverse da Windows ---