fix(asio): keep std::println usable with import asio

[wellwei]

Summary

• Repair the Form B chriskohlhoff.asio@1.38.1 module recipe by propagating Asio's supported ASIO_NO_IOSTREAM define through the default separate-compilation feature.
• Keep libc++ <print> out of the asio BMI so std::println remains usable after import std; import fmt; import asio;.
• Extend the active asio-module consumer test with the reported import combination and both zero-argument and formatted std::println calls.

Root cause

The Asio module wrapper includes the TCP surface. Without ASIO_NO_IOSTREAM, that path also exposes Asio's iostream adapters and pulls libc++ <print> into the asio BMI. With the same standard-library declarations already provided by the std BMI, Clang 22.1.8 can bind std::println incorrectly and reports failures such as:

no member named 'parse' in 'std::__1::formatter<std::__1::basic_format_string<char>>'

Defining ASIO_NO_IOSTREAM at the existing separate-compilation feature boundary keeps the BMI, Asio implementation sources, and consumer TUs on one consistent macro contract.

Compatibility

This disables Asio's iostream/streambuf adapter surface, including basic_socket_iostream (asio::ip::tcp::iostream) and streambuf-based read_at/write_at overloads. Core socket, timer, coroutine, and asynchronous APIs remain available and continue to be exercised by the existing consumer tests.

This is a recipe repair only:

• upstream source URLs and SHA-256 values are unchanged;
• Linux, macOS, and Windows declarations are unchanged;
• index.toml's min_mcpp and the workflow mcpp pin are unchanged;
• no mirror or release asset changes are required.

Verification

Reproduced before the repair on macOS with mcpp 2026.8.6.2, LLVM 22.1.8, and import std; import fmt; import asio;: the consumer failed in libc++ format_functions.h with the formatter diagnostics above.

After the repair:

• mcpp test --cache=local from tests/examples/asio-module: 5 passed, 0 failed; the surface binary printed both expected std::println lines.
• MCPP_TIMINGS=/tmp/mcpp-index-asio-pr-timings.tsv bash tests/run_members.sh asio-module: 5 passed, 0 failed.
• Lua syntax, mirror URL, and package-name checks passed locally with Lua 5.5.0.
• mcpp xpkg parse pkgs/c/chriskohlhoff.asio.lua passed with mcpp 2026.8.6.2.
• git diff --check upstream/main..HEAD passed.

Local behavioral coverage is macOS ARM64 only. The PR's required GitHub Actions matrix must provide Linux, macOS, and Windows evidence before merge.

[wellwei]

CI follow-up

The platform matrix exercised both affected members selected by the workflow (asio-module and asio-ssl):

• macOS: both members passed; asio-module printed both regression lines and reported 5/5 tests passed.
• Windows: both members passed; asio-module printed both regression lines and reported 5/5 tests passed.
• Linux: asio-module passed with 5/5 tests and both regression lines. The same job then failed while building the unrelated compat.openssl@3.5.1 dependency for asio-ssl; GCC reported missing stdlib.h, limits.h, and assert.h.

This OpenSSL failure is reproducible on the current base branch: the recent main validation run 31102350743 has the same missing-system-header failure in two Linux workspace shards. I attempted to rerun the failed job, but the upstream repository requires admin rights for reruns; the fork account does not have that permission.