[Sunrisepeak]

Implements mcpp-community/mcpp design 2026-11-09 (the target model PR): mcpp's unit tests move from GoogleTest to mtest, and nine of them read what mcpp prints on its standard error, which GoogleTest's CaptureStderr gave them.

Changes

• stdio::capture. capture::begin(stream::error) (or stream::output) makes the stream a file until text() or destruction puts it back; text() gives what was written meanwhile — through openkal, the C library and the C++ streams, and by programs started meanwhile.
• Each system its own descriptor. Linux and macOS: the kernel's (openat/open, dup, dup2), as output_to_error does; Windows under the C runtime: _wopen and _dup2, which also sets the standard handle; Windows under openkal-musl: the C library's descriptor and the standard handle, each appending to the file; Emscripten: the C library's; an image: not_supported.
• process::resources. of::self: this program's user and system time, largest resident set, and bytes read and written (Linux /proc/self/io, Windows GetProcessIoCounters); of::children: its waited-for children together, by getrusage (Linux and macOS). Windows children and images answer not_supported. mtest's resource monitor (design 2026-11-09 §17.5) measures a program by starting it from a process of its own.
• Test. tests/process_test captures the error stream of a copy of itself written through openkal, the C library and a started program, then checks that the stream was put back; a copy that touches 64 MiB is accounted to its parent.

Verification

• Linux x86_64: mcpp test process passes (the new check holds).

Co-authored-by: speak-agent [email protected]