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/progressfunc.c

@@ -27,10 +27,11 @@
  * </DESC>
  */
 #include <stdio.h>
+
 #include <curl/curl.h>
 
-#define MINIMAL_PROGRESS_FUNCTIONALITY_INTERVAL     3000000
-#define STOP_DOWNLOAD_AFTER_THIS_MANY_BYTES         6000
+#define MINIMAL_PROGRESS_FUNCTIONALITY_INTERVAL 3000000
+#define STOP_DOWNLOAD_AFTER_THIS_MANY_BYTES     6000
 
 struct myprogress {
   curl_off_t lastruntime; /* type depends on version, see above */
@@ -53,14 +54,19 @@ static int xferinfo(void *p,
      be used */
   if((curtime - myp->lastruntime) >= MINIMAL_PROGRESS_FUNCTIONALITY_INTERVAL) {
     myp->lastruntime = curtime;
-    fprintf(stderr, "TOTAL TIME: %lu.%06lu\r\n",
-            (unsigned long)(curtime / 1000000),
-            (unsigned long)(curtime % 1000000));
+    fprintf(stderr, "TOTAL TIME: %" CURL_FORMAT_CURL_OFF_T
+            ".%06" CURL_FORMAT_CURL_OFF_T "\r\n",
+            curtime / 1000000,
+            curtime % 1000000);
   }
 
-  fprintf(stderr, "UP: %lu of %lu  DOWN: %lu of %lu\r\n",
-          (unsigned long)ulnow, (unsigned long)ultotal,
-          (unsigned long)dlnow, (unsigned long)dltotal);
+  fprintf(stderr,
+          "UP: "
+          "%" CURL_FORMAT_CURL_OFF_T " of %" CURL_FORMAT_CURL_OFF_T "  "
+          "DOWN: "
+          "%" CURL_FORMAT_CURL_OFF_T " of %" CURL_FORMAT_CURL_OFF_T "\r\n",
+          ulnow, ultotal,
+          dlnow, dltotal);
 
   if(dlnow > STOP_DOWNLOAD_AFTER_THIS_MANY_BYTES)
     return 1;
@@ -70,11 +76,15 @@ static int xferinfo(void *p,
 int main(void)
 {
   CURL *curl;
-  CURLcode res = CURLE_OK;
-  struct myprogress prog;
+
+  CURLcode result = curl_global_init(CURL_GLOBAL_ALL);
+  if(result != CURLE_OK)
+    return (int)result;
 
   curl = curl_easy_init();
   if(curl) {
+    struct myprogress prog;
+
     prog.lastruntime = 0;
     prog.curl = curl;
 
@@ -85,13 +95,14 @@ int main(void)
     curl_easy_setopt(curl, CURLOPT_XFERINFODATA, &prog);
 
     curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 0L);
-    res = curl_easy_perform(curl);
+    result = curl_easy_perform(curl);
 
-    if(res != CURLE_OK)
-      fprintf(stderr, "%s\n", curl_easy_strerror(res));
+    if(result != CURLE_OK)
+      fprintf(stderr, "%s\n", curl_easy_strerror(result));
 
     /* always cleanup */
     curl_easy_cleanup(curl);
   }
-  return (int)res;
+  curl_global_cleanup();
+  return (int)result;
 }