Write the native MSVC result report when a prerequisite probe times out

Start in tools/check_msvc_native.py at Audit.run and review how prerequisite probe failures are handled after Audit.command records timeout evidence. Add mocked regression coverage in tools/test_check_msvc_native.py and run python3 -m unittest tools.test_check_msvc_native; done means either version-probe timeout writes a failed result.json while preserving commands.json and prevents scenario compilation.

Problem

In Audit.run, the initial clang-cl --version and wavec -V probes run outside Audit.case. Audit.command records a timeout and rethrows subprocess.TimeoutExpired, but the outer exception handler does not catch that exception. Execution skips the result.json write.

This leaves commands.json with useful evidence but no final machine-readable failure summary. The job correctly fails; this issue is about preserving its report, not changing timeout success semantics.

Reproduction

Reproduced on master 0c32b946 without Windows or an installed SDK, by injecting a prerequisite timeout:

import argparse
import subprocess
import tempfile
from pathlib import Path
from unittest.mock import patch
from tools.check_msvc_native import Audit

with tempfile.TemporaryDirectory() as temporary:
    root = Path(temporary)
    audit = Audit(argparse.Namespace(
        output=root / 'audit', wavec=root / 'wavec.exe',
        llvm_bin=root / 'llvm/bin', target='aarch64-pc-windows-msvc'))
    with patch('tools.check_msvc_native.require_native'), patch(
        'tools.check_msvc_native.run_process',
        side_effect=subprocess.TimeoutExpired(
            ['clang-cl.exe', '--version'], 90, output=b'partial probe')):
        try:
            audit.run()
        except subprocess.TimeoutExpired:
            pass
    print((audit.output / 'commands.json').exists())  # True
    print((audit.output / 'result.json').exists())    # False: should be True

Bounded change

Difficulty: easy. Suitable for a first contribution: timeout recording already works inside scenario execution. Extend prerequisite failure handling so the final report is written before propagating a failure.

Acceptance:

• Timeouts in either version probe produce result.json with passed: false, the selected target, and a prerequisite failure.
• Preserve timeout duration, command identity and partial output in commands.json.
• Do not start scenario compilation after prerequisite failure and do not convert it into a skip/pass.
• Add fast mocked regression tests in tools/test_check_msvc_native.py; no 90-second wait or Windows host is required.
• Existing missing-tool and per-scenario failure behavior stays covered.

Validation command: python3 -m unittest tools.test_check_msvc_native.