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_platform_win.h

@@ -75,6 +75,7 @@ static inline void* el_win_dlsym(void* handle, const char* name) {
 #include <direct.h>            /* _mkdir */
 #define mkdir(path, mode) _mkdir(path)     /* POSIX mkdir(path,mode) → _mkdir(path) */
 #define timegm _mkgmtime                   /* UTC tm → time_t */
+#define fsync(fd) _commit(fd)              /* no fsync() on Windows; _commit() (<io.h>) is the equiv */
 
 /* setenv/unsetenv: not in the Windows CRT; map to _putenv_s / SetEnvironmentVariable. */
 static inline int setenv(const char* name, const char* value, int overwrite) {