Fix CI: gate POSIX struct iovec asserts on !_WIN32

MSYS2/mingw does not ship <sys/uio.h>, so the layout-pinning asserts
that compare httpserver::iovec_entry against POSIX struct iovec must be
gated. The MHD_IoVec asserts stay unconditional — that's the type the
dispatch path actually casts to, and libmicrohttpd ships its own portable
MHD_IoVec definition.

Files:
- src/iovec_response.cpp: wrap <sys/uio.h> include and the four POSIX
  struct iovec asserts (size/base/len/alignof) in #ifndef _WIN32.
- src/details/body.cpp: same — body.cpp duplicates the iovec_response.cpp
  asserts during the M2 transition.
- src/httpserver/details/body.hpp: drop <sys/uio.h> include outright;
  body.hpp uses iovec_entry (the portable replacement) and MHD_IoVec
  but never references POSIX struct iovec.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: etr

src/details/body.cpp

@@ -24,8 +24,10 @@
 #include <microhttpd.h>
 #include <sys/stat.h>
 #include <sys/types.h>
-#include <sys/uio.h>
 #include <unistd.h>
+#ifndef _WIN32
+#include <sys/uio.h>        // POSIX struct iovec — used for layout-pin asserts
+#endif
 
 #include <cstddef>
 #include <cstdint>
@@ -47,7 +49,12 @@ namespace detail {
 //
 // LIBHTTPSERVER_TODO_TASK013: drop the originals from iovec_response.cpp
 // when iovec_response is removed.
+//
+// The POSIX `struct iovec` asserts are gated on !_WIN32 (no <sys/uio.h> on
+// MSYS2/mingw); the MHD_IoVec asserts run everywhere because that's the
+// type the dispatch path actually casts to.
 // ---------------------------------------------------------------------------
+#ifndef _WIN32
 static_assert(sizeof(::httpserver::iovec_entry) == sizeof(struct iovec),
     "iovec_entry size must match POSIX struct iovec — divergent platform; "
     "implement memcpy fallback (see TASK-004)");
@@ -57,6 +64,10 @@ static_assert(offsetof(::httpserver::iovec_entry, base) ==
 static_assert(offsetof(::httpserver::iovec_entry, len) ==
                   offsetof(struct iovec, iov_len),
     "iovec_entry::len offset must match struct iovec::iov_len");
+static_assert(alignof(::httpserver::iovec_entry) == alignof(struct iovec),
+    "iovec_entry alignment must match POSIX struct iovec — divergent platform; "
+    "implement memcpy fallback (see TASK-004)");
+#endif  // !_WIN32
 
 static_assert(sizeof(::httpserver::iovec_entry) == sizeof(MHD_IoVec),
     "iovec_entry size must match libmicrohttpd MHD_IoVec — MHD layout drift");
@@ -66,10 +77,6 @@ static_assert(offsetof(::httpserver::iovec_entry, base) ==
 static_assert(offsetof(::httpserver::iovec_entry, len) ==
                   offsetof(MHD_IoVec, iov_len),
     "iovec_entry::len offset must match MHD_IoVec::iov_len");
-
-static_assert(alignof(::httpserver::iovec_entry) == alignof(struct iovec),
-    "iovec_entry alignment must match POSIX struct iovec — divergent platform; "
-    "implement memcpy fallback (see TASK-004)");
 static_assert(alignof(::httpserver::iovec_entry) == alignof(MHD_IoVec),
     "iovec_entry alignment must match MHD_IoVec — MHD layout drift");
 

src/httpserver/details/body.hpp

@@ -37,7 +37,6 @@
 
 #include <microhttpd.h>
 #include <sys/types.h>      // ssize_t
-#include <sys/uio.h>        // private header may include POSIX scatter/gather
 
 #include <cassert>
 #include <cstddef>

src/iovec_response.cpp

@@ -21,7 +21,9 @@
 #include "httpserver/iovec_response.hpp"
 
 #include <microhttpd.h>
-#include <sys/uio.h>
+#ifndef _WIN32
+#include <sys/uio.h>        // POSIX struct iovec — used for layout-pin asserts
+#endif
 
 #include <cstddef>
 #include <limits>
@@ -52,7 +54,12 @@ namespace httpserver {
 // asserts are the gate — a build failure on the divergent platform is
 // the desired outcome (loud, immediate, with the assert string naming
 // what diverged).
+//
+// The POSIX `struct iovec` asserts are gated on !_WIN32: MSYS2/mingw does
+// not ship <sys/uio.h>. The MHD_IoVec asserts are unconditional — that's
+// the type the dispatch path actually casts to.
 // ---------------------------------------------------------------------------
+#ifndef _WIN32
 static_assert(sizeof(::httpserver::iovec_entry) == sizeof(struct iovec),
     "iovec_entry size must match POSIX struct iovec — divergent platform; "
     "implement memcpy fallback (see TASK-004)");
@@ -62,6 +69,10 @@ static_assert(offsetof(::httpserver::iovec_entry, base) ==
 static_assert(offsetof(::httpserver::iovec_entry, len) ==
                   offsetof(struct iovec, iov_len),
     "iovec_entry::len offset must match struct iovec::iov_len");
+static_assert(alignof(::httpserver::iovec_entry) == alignof(struct iovec),
+    "iovec_entry alignment must match POSIX struct iovec — divergent platform; "
+    "implement memcpy fallback (see TASK-004)");
+#endif  // !_WIN32
 
 static_assert(sizeof(::httpserver::iovec_entry) == sizeof(MHD_IoVec),
     "iovec_entry size must match libmicrohttpd MHD_IoVec — MHD layout drift");
@@ -75,9 +86,6 @@ static_assert(offsetof(::httpserver::iovec_entry, len) ==
 // Alignment pinning: ensures the reinterpret_cast array stride is safe on
 // architectures that trap on misaligned loads (SPARC, some ARM configs).
 // CWE-704: without alignof equality the cast is UB even when size/offset match.
-static_assert(alignof(::httpserver::iovec_entry) == alignof(struct iovec),
-    "iovec_entry alignment must match POSIX struct iovec — divergent platform; "
-    "implement memcpy fallback (see TASK-004)");
 static_assert(alignof(::httpserver::iovec_entry) == alignof(MHD_IoVec),
     "iovec_entry alignment must match MHD_IoVec — MHD layout drift");