runtime: make Windows soul reproducible from a clean el checkout

Two el_runtime portability defects only ever lived in staged local copies
used to hand-build neuron-ui PR #136's curl-enabled Windows neuron.exe.
gcc 15 promotes both to hard errors, so a clean el checkout cannot rebuild
that soul. Upstream the minimal fixes so the build is reproducible:

- http_serve_async: cast setsockopt optval to (const char*). Win32/mingw
  setsockopt wants const char*, not int*; the cast is a no-op on POSIX and
  matches the four already-cast sites elsewhere in this file.
- engram_save persist path: map fsync -> _commit in the _WIN32-only
  el_platform_win.h (io.h already included). Windows has no fsync(); the
  POSIX path is untouched.

lang/el-compiler/runtime/el_runtime.c

@@ -1963,8 +1963,9 @@ void http_serve_async(el_val_t port, el_val_t handler) {
     int sock = socket(AF_INET6, SOCK_STREAM, 0);
     if (sock < 0) { perror("socket"); return; }
     int yes = 1; int no = 0;
-    setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes));
-    setsockopt(sock, IPPROTO_IPV6, IPV6_V6ONLY, &no, sizeof(no));
+    /* Win32/mingw setsockopt takes optval as (const char*); the cast is portable on POSIX too. */
+    setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, (const char*)&yes, sizeof(yes));
+    setsockopt(sock, IPPROTO_IPV6, IPV6_V6ONLY, (const char*)&no, sizeof(no));
     struct sockaddr_in6 addr;
     memset(&addr, 0, sizeof(addr));
     addr.sin6_family = AF_INET6;