Windows tree tests start their ping through .NET: the cancel test no longer waits for PowerShell to read every module

[undistracted2cha]

sys::tests::windows_cancel_kills_the_whole_tree failed at random on Windows ("the ping never started", 3 of ~60 runs: 38852822160, 38816142679, 38745678952).

Cause (measured on fresh windows-latest VMs)

• The runner image sets PSModuleAnalysisCachePath=C:\PSModuleAnalysisCachePath\ModuleAnalysisCache, a prebuilt module cache. The cancel test starts PowerShell through run_isolated with a short environment, which drops it, and the per-user default cache does not exist on the runner.
• Before its first cmdlet (Start-Process), Windows PowerShell then reads every module on its path (Az 15.6.1 and the rest). Stack of the stuck PowerShell: CommandDiscovery.TryModuleAutoDiscovery → AnalysisCache.AnalyzeManifestModule → Get-Module -ListAvailable → LoadBinaryModule.
• Finding Start-Process: 22–60 s alone, 12–75 s while the other tests run; 0.1 s with the variable passed through. PowerShell saves its cache only seconds after the analysis, so a PowerShell the test kills right away never saves it: every run paid again. Past 60 s the watcher panicked without cancelling, and the test sat on until run_isolated's 120 s limit.

Not a run_isolated defect: it passes exactly the environment it is given. On a user's PC the default cache is in the profile and warm, and the app's isolated program (Claude Code) is not PowerShell.

Change (tests only)

• The tree script uses .NET calls only (Process.Start with ShellExecute and a hidden window, File.WriteAllText): no module discovery, whatever the environment.
• wait_for_pid returns None instead of panicking. The cancel test cancels in every case, and both tree tests fail with PowerShell's output, its exit code and the elapsed time.

Proof (temporary workflow on this branch, removed in the last commit)

• 65 full cargo test --lib runs on 5 fresh VMs, no failure (38860347014).
• Suite without the 20 s timeout test: 3.3 s at most; cancel test alone: 0.6 s (30 + 30 runs, 38861180600). Before: 12–75 s for the cancel test (38855348805, 38858791739).