[DO NOT MERGE] measurement for #693: which process exits, and the encoding build.ninja is written in

[Sunrisepeak]

Temporary measurement branch. Do not merge. Every workflow except measure-693.yml is removed on this branch so that only these readings run.

Readings for #693 on windows-latest (system ANSI code page 1252), with the released mcpp 2026.9.25.1:

• Q1. Which process ends with 0xC0000409 in a directory outside the ANSI code page: mcpp.exe itself, the release zip's mcpp.bat, or the xlings shim that fronts mcpp in an xlings installation. Windows Error Reporting events and local dumps name the faulting image and module.
• Q2. Whether build.ninja and compile_commands.json are written in the process ANSI code page while the Ninja that mcpp runs reads UTF-8. Tested in a directory the code page can represent (caf + U+00E9), where the two encodings differ in bytes.
• Q3. Whether a UTF-8 activeCodePage manifest, embedded with mt.exe into a copy of the released mcpp.exe, changes Q1 and Q2, and whether it is sufficient end to end: clang 20.1.7, lld, Ninja, the std module, and a build.mcpp written with narrow APIs or with wide APIs.

The readings feed .agents/docs/2026-09-25-issues-693-696-triage-and-repair-plan.md. This PR is closed once they are recorded.

[Sunrisepeak]

Readings recorded in .agents/docs/2026-09-25-issues-693-696-triage-and-repair-plan.md (§6.2 and Appendix A.10). Closing; the branch stays for the script it references.

Run	What it measured
36162457075	Q1 (which process exits). The build readings of this run are void: the PowerShell loop variable $fx overwrote $Fx, so no build started.
36163057371	Q1 with the shim inside its workspace, cdb on xlings --version, Q2/Q3 on the llvm row
36163850821	the MSVC and MinGW rows under a UTF-8 activeCodePage copy of mcpp.exe; which MinGW executables declare UTF-8
36164076852	a non-ASCII MCPP_HOME; replicates every earlier reading

Findings:

• The silent 0xC0000409 comes from xlings: its start-up throws std::system_error ("No mapping for the Unicode character exists in the target multi-byte code page.") in a working directory outside the ANSI code page, and its main has no exception boundary. The released mcpp.exe prints its version in the same directory.
• mcpp 2026.9.25.1 fails every build, with an internal exception, when the project or the mcpp home lies under a non-ASCII name. This includes names that the ANSI code page can represent: [json.exception.type_error.316] invalid UTF-8 byte.
• With a UTF-8 activeCodePage manifest, mcpp.exe builds and runs the fixtures in both kinds of non-ASCII directory on the llvm, MSVC and MinGW rows. A build program has to be in UTF-8 as well.

[Sunrisepeak]

Round 5 and 5b recorded; closed again.