fix: Windows toolchain compatibility (curl 8.21 re-vendor, endless reconfigure loop) (#4355)

## Problem

Windows builds break with a current local toolchain (Scoop LLVM 22.1.8,
CMake 4.4.0, VS 2026), in two independent ways:

1. The build stops at curl's deliberate guard: `#error "no non-blocking
method was found/used/set"` in `third-party/curl/lib/nonblock.c`.
2. From the second configure onward, `cmake --build` re-runs CMake in an
endless loop (observed 42 consecutive reconfigure cycles in a single
build). Likely the same mechanism behind the "endlessly building" VS
2026 note in `docs/setup/dev/vs.md`.

## Root cause

1. `third-party/curl/CMake/CurlTests.c` passes `int *` to
`ioctlsocket()`, whose third parameter is `u_long *`. Clang 22 promotes
`-Wincompatible-pointer-types` to a hard error in C, so the
`HAVE_IOCTLSOCKET_FIONBIO` try_compile silently fails and
`curl_config.h` never defines it. Upstream CI does not see this because
the windows-2022 runner image ships an older LLVM. GCC 14 promotes the
same warning to a hard error, which is very likely the `CurlTests.c.obj`
failure reported from MSYS2 in open-goal/jak-project#3551. Upstream curl
hit the identical problem with GCC 14 and fixed the probe in curl 8.8.0
(curl/curl#13578).
2. The root CMakeLists copies the build tree's `compile_commands.json`
into `<src>/build/` for clangd using `configure_file()`, which registers
its input as a configure dependency. CMake rewrites
`compile_commands.json` late in every generation, after
`CTestTestfile.cmake` and `cmake_install.cmake` (outputs of the same
Ninja regen rule), so once the dependency is registered the rule is
deterministically dirty and every `ninja` invocation re-runs CMake. A
pristine first configure is safe (the file does not exist yet, so the
`if(EXISTS ...)` guard skips the copy), which is why the loop looks
machine- or IDE-specific.

## Fix

1. Per review, re-vendor `third-party/curl` at the `curl-8_21_0` tag
(previously `curl-8_3_0`), which carries the upstream probe fix plus two
years of upstream development; `vendor.yaml` updated to match.
Adjustments the version jump forced:
- curl 8.15 removed the native macOS Secure Transport backend
(`CURL_USE_SECTRANSP`), so macOS now builds curl against OpenSSL like
Linux. The two macOS workflows install Homebrew `openssl@3` and export
`OPENSSL_ROOT_DIR` (keg-only), and the macOS setup docs gained the same
two lines.
- `CURL_BROTLI` / `CURL_ZSTD` switched to AUTO-detection in curl 8.10;
pinned OFF to keep the previous no-compression behavior and avoid
silently linking whatever the CI images happen to have.
- curl's new top-level `BUILD_EXAMPLES` cache option (default ON) leaked
into discord-rpc's identically named option and broke configure at a
nonexistent `examples/send-presence` directory; pinned OFF ahead of the
third-party subdirectories.

The diff is dominated by the mechanical tag-tree swap under
`third-party/curl` (linguist-vendored, collapsed in review). The
hand-written changes are `CMakeLists.txt`, the two macOS workflows,
`docs/setup/system/macos.md`, and `vendor.yaml`.
2. Swap `configure_file()` for `file(COPY ...)`: the same clangd copy
with no configure dependency registered. (`file(COPY_FILE ...
ONLY_IF_DIFFERENT)` would be cleaner still but requires CMake 3.21,
above the declared `cmake_minimum_required(VERSION 3.10)`.)

## Test plan

- [x] Fresh `cmake --preset Release-windows-clang` (LLVM 22, no cache
seeding) completes and logs `Enabled SSL backends: Schannel`; the
FIONBIO probe passes without the previous `#error`
- [x] Full Windows Release build from scratch in the branch worktree
(all 1422 targets)
- [x] goalc-test suite: 1509 passed, 0 failed
- [x] Second consecutive configure with `compile_commands.json` present:
the regen rule in `build.ninja` has no `compile_commands.json` input;
`<src>/build/compile_commands.json` is still refreshed for clangd
- [x] Repeated `ninja` invocations after a full build no longer re-run
CMake
- [x] macOS Intel and ARM CI green (first exercise of the OpenSSL
backend switch)

---

I work off a self-hosted forge, so this GitHub account is quiet; the
configure logs and ninja dirty-node traces from the investigation are
available if anyone wants the raw data.

(AI-assisted)

third-party/curl/docs/examples/multi-debugcallback.c

@@ -25,20 +25,12 @@
  * multi interface and debug callback
  * </DESC>
  */
-
 #include <stdio.h>
 #include <string.h>
 
-/* somewhat unix-specific */
-#include <sys/time.h>
-#include <unistd.h>
-
-/* curl stuff */
 #include <curl/curl.h>
 
-#define TRUE 1
-
-static void dump(const char *text, FILE *stream, unsigned char *ptr,
+static void dump(const char *text, const unsigned char *ptr,
                  size_t size, char nohex)
 {
   size_t i;
@@ -50,20 +42,20 @@ static void dump(const char *text, FILE *stream, unsigned char *ptr,
     /* without the hex output, we can fit more on screen */
     width = 0x40;
 
-  fprintf(stream, "%s, %10.10lu bytes (0x%8.8lx)\n",
+  fprintf(stderr, "%s, %lu bytes (0x%lx)\n",
           text, (unsigned long)size, (unsigned long)size);
 
-  for(i = 0; i<size; i += width) {
+  for(i = 0; i < size; i += width) {
 
-    fprintf(stream, "%4.4lx: ", (unsigned long)i);
+    fprintf(stderr, "%4.4lx: ", (unsigned long)i);
 
     if(!nohex) {
       /* hex not disabled, show it */
       for(c = 0; c < width; c++)
         if(i + c < size)
-          fprintf(stream, "%02x ", ptr[i + c]);
+          fprintf(stderr, "%02x ", ptr[i + c]);
         else
-          fputs("   ", stream);
+          fputs("   ", stderr);
     }
 
     for(c = 0; (c < width) && (i + c < size); c++) {
@@ -73,37 +65,31 @@ static void dump(const char *text, FILE *stream, unsigned char *ptr,
         i += (c + 2 - width);
         break;
       }
-      fprintf(stream, "%c",
-              (ptr[i + c] >= 0x20) && (ptr[i + c]<0x80)?ptr[i + c]:'.');
-      /* check again for 0D0A, to avoid an extra \n if it's at width */
+      fprintf(stderr, "%c",
+              (ptr[i + c] >= 0x20) && (ptr[i + c] < 0x80) ? ptr[i + c] : '.');
+      /* check again for 0D0A, to avoid an extra \n if it is at width */
       if(nohex && (i + c + 2 < size) && ptr[i + c + 1] == 0x0D &&
          ptr[i + c + 2] == 0x0A) {
         i += (c + 3 - width);
         break;
       }
     }
-    fputc('\n', stream); /* newline */
+    fputc('\n', stderr); /* newline */
   }
-  fflush(stream);
 }
 
-static
-int my_trace(CURL *handle, curl_infotype type,
-             unsigned char *data, size_t size,
-             void *userp)
+static int my_trace(CURL *curl, curl_infotype type,
+                    char *data, size_t size, void *userp)
 {
   const char *text;
 
   (void)userp;
-  (void)handle; /* prevent compiler warning */
+  (void)curl;
 
   switch(type) {
   case CURLINFO_TEXT:
     fprintf(stderr, "== Info: %s", data);
-    /* FALLTHROUGH */
-  default: /* in case a new one is introduced to shock us */
     return 0;
-
   case CURLINFO_HEADER_OUT:
     text = "=> Send header";
     break;
@@ -116,51 +102,64 @@ int my_trace(CURL *handle, curl_infotype type,
   case CURLINFO_DATA_IN:
     text = "<= Recv data";
     break;
+  default: /* in case a new one is introduced to shock us */
+    return 0;
   }
 
-  dump(text, stderr, data, size, TRUE);
+  dump(text, (const unsigned char *)data, size, 1);
   return 0;
 }
 
 /*
- * Simply download an HTTP file.
+ * Download an HTTP file.
  */
 int main(void)
 {
-  CURL *http_handle;
-  CURLM *multi_handle;
+  CURL *curl;
 
-  int still_running = 0; /* keep number of running handles */
+  CURLcode result = curl_global_init(CURL_GLOBAL_ALL);
+  if(result != CURLE_OK)
+    return (int)result;
 
-  http_handle = curl_easy_init();
+  curl = curl_easy_init();
+  if(curl) {
 
-  /* set the options (I left out a few, you will get the point anyway) */
-  curl_easy_setopt(http_handle, CURLOPT_URL, "https://www.example.com/");
+    CURLM *multi;
 
-  curl_easy_setopt(http_handle, CURLOPT_DEBUGFUNCTION, my_trace);
-  curl_easy_setopt(http_handle, CURLOPT_VERBOSE, 1L);
+    /* set the options (I left out a few, you get the point anyway) */
+    curl_easy_setopt(curl, CURLOPT_URL, "https://www.example.com/");
 
-  /* init a multi stack */
-  multi_handle = curl_multi_init();
+    curl_easy_setopt(curl, CURLOPT_DEBUGFUNCTION, my_trace);
+    curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
 
-  /* add the individual transfers */
-  curl_multi_add_handle(multi_handle, http_handle);
+    /* init a multi stack */
+    multi = curl_multi_init();
+    if(multi) {
 
-  do {
-    CURLMcode mc = curl_multi_perform(multi_handle, &still_running);
+      int still_running = 0; /* keep number of running handles */
 
-    if(still_running)
-      /* wait for activity, timeout or "nothing" */
-      mc = curl_multi_poll(multi_handle, NULL, 0, 1000, NULL);
+      /* add the individual transfers */
+      curl_multi_add_handle(multi, curl);
 
-    if(mc)
-      break;
+      do {
+        CURLMcode mresult = curl_multi_perform(multi, &still_running);
 
-  } while(still_running);
+        if(still_running)
+          /* wait for activity, timeout or "nothing" */
+          mresult = curl_multi_poll(multi, NULL, 0, 1000, NULL);
 
-  curl_multi_cleanup(multi_handle);
+        if(mresult)
+          break;
 
-  curl_easy_cleanup(http_handle);
+      } while(still_running);
+
+      curl_multi_cleanup(multi);
+    }
+
+    curl_easy_cleanup(curl);
+  }
+
+  curl_global_cleanup();
 
   return 0;
 }