fix(asio): pin ASIO_HAS_THREADS for the llvm-on-Windows toolchain

With the symlink-free archive in place, the Windows job progressed to
runtime failures: core and network crashed with 0xC0000409 and coroutine
exited 1 while the thread-free tests (experimental/platform/surface)
passed. Reproduced under mingw+wine: asio's thread detection keys off CRT
macros (_MT/_REENTRANT/_POSIX_THREADS) the toolchain does not define, so
it silently selects null_thread and the steady_timer wait thread throws
operation_not_supported (10045, 'thread: asio.system error').

Pin ASIO_HAS_THREADS in the default standalone feature. asio only tests
defined(ASIO_HAS_THREADS) and POSIX pthread selection still runs beneath
it, so this is a no-op where detection already works. All six consumer
tests pass under mingw+wine with the pin (platform's pipe path is a wine
limitation, passes on real Windows), and the Linux member suite passes
6/6 cold with the define present in every consumer TU's compile command.

Author: speak-agent

.agents/docs/2026-07-17-add-asio-plan.md

@@ -84,7 +84,18 @@ The default `standalone` feature contributes these public preprocessor defines:
 
 - `ASIO_STANDALONE`;
 - `ASIO_HEADER_ONLY`;
-- `ASIO_DISABLE_BOOST_CONTEXT_FIBER`.
+- `ASIO_DISABLE_BOOST_CONTEXT_FIBER`;
+- `ASIO_HAS_THREADS` — asio's own thread detection keys off CRT macros
+  (`_MT`/`_REENTRANT`/`_POSIX_THREADS`) that the workspace's llvm-on-Windows
+  toolchain does not define. Without the pin asio silently selects
+  `null_thread` and every internal-thread operation (notably the
+  waitable-timer wait thread behind `steady_timer`) throws
+  `operation_not_supported` (10045) at runtime — observed as the core,
+  coroutine, and network consumer tests failing on the Windows runner while
+  the thread-free tests passed. asio only ever tests
+  `defined(ASIO_HAS_THREADS)`, and the POSIX pthread selection still runs
+  beneath it, so the pin is a no-op on platforms where detection already
+  works (re-verified by the Linux consumer suite).
 
 mcpp 0.0.94 accepts feature `defines` in the current xpkg parser and propagates
 them to consumers. The Asio surface test rejects builds where any of these

pkgs/c/compat.asio.lua

@@ -63,13 +63,23 @@ int mcpp_compat_asio_headers_anchor(void) { return 0; }
         targets = { ["asio"] = { kind = "lib" } },
         -- Explicitly pin the package's public configuration. `standalone` is a
         -- default feature so its defines propagate to every consumer TU.
+        --
+        -- ASIO_HAS_THREADS: asio's own thread detection keys off CRT macros
+        -- (_MT/_REENTRANT/_POSIX_THREADS) that the workspace's llvm-on-Windows
+        -- toolchain does not define, so asio silently selects null_thread and
+        -- every internal-thread operation (e.g. the waitable-timer wait thread)
+        -- throws operation_not_supported (10045) at runtime. All supported
+        -- targets are multithreaded; pin the detection result. asio only ever
+        -- tests defined(ASIO_HAS_THREADS), and on POSIX the pthread selection
+        -- below it still runs, so this is a no-op where detection already works.
         features = {
             ["default"] = { implies = { "standalone" } },
             ["standalone"] = {
                 defines = {
                     "ASIO_STANDALONE",
                     "ASIO_HEADER_ONLY",
                     "ASIO_DISABLE_BOOST_CONTEXT_FIBER",
+                    "ASIO_HAS_THREADS",
                 },
             },
         },