Remove 32-bit limitation (#2388)

* Remove 32-bit limitation

* Fix build problems

* Add 32-bit disclaimer and fix MSVC x86 warnings

- Move 32-bit warning to top of README with strong disclaimer
- Add static_cast<size_t> to fix truncation warnings on 32-bit MSVC

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

test/test_32bit_build.cpp

@@ -0,0 +1,9 @@
+#include "../httplib.h"
+
+int main() {
+  httplib::Server svr;
+  httplib::Client cli("localhost", 8080);
+  (void)svr;
+  (void)cli;
+  return 0;
+}