debug_server: use _DARWIN_C_SOURCE on macOS for full BSD+POSIX namespace

On macOS, defining _POSIX_C_SOURCE restricts the visible namespace and HIDES
BSD symbols the TCP debug server needs — notably INADDR_LOOPBACK (and
struct timeval). clang then fails with 'use of undeclared identifier
INADDR_LOOPBACK'. Use _DARWIN_C_SOURCE on Apple (which exposes the full
BSD+POSIX namespace incl. clock_gettime), keeping _POSIX_C_SOURCE for glibc.

Mirrors the same __APPLE__/_DARWIN_C_SOURCE guard already used in psx_fiber.c.

runtime/src/debug_server.c

@@ -9,7 +9,9 @@
  */
 /* Expose POSIX clock_gettime()/CLOCK_MONOTONIC (used by monotonic_ms) on
  * glibc — must precede any system header. Harmless on Windows/macOS. */
-#ifndef _POSIX_C_SOURCE
+#ifdef __APPLE__
+#  define _DARWIN_C_SOURCE 1   /* full BSD+POSIX namespace: clock_gettime AND INADDR_LOOPBACK/timeval */
+#elif !defined(_POSIX_C_SOURCE)
 #  define _POSIX_C_SOURCE 200809L
 #endif
 #include <time.h>