Fix Windows job cancellation and stabilize Cursor pipe test

[sox8502]

Windows local runs now enter a native Job Object before their launcher starts, so cancellation terminates the whole tree and remains available to a separate supervisor. Existing runs retain the previous cancellation path. Completed exit codes are preserved.

The Cursor stdin test keeps its Windows .cmd launcher but uses Git Bash cat instead of starting PowerShell. The original timeout reproduced in 1 of 3 Windows runs with 16 test threads; a separate startup probe demonstrated children surviving the old taskkill /T cancellation. Lifecycle tests now use UUID scratch directories so reused process IDs cannot pick up stale exit codes.

Verification:

• Both targeted tests repeated 20 times on Windows after the production fix; lifecycle test repeated another 20 times after fixture isolation
• Windows Clippy and full suite: 824 passed, 2 ignored, including a final run with 16 test threads
• Job-name lifetime, background-child cancellation, and completed-exit-code regression checks
• macOS formatting, Clippy, locked debug/release builds, 862 Rust tests, and development build-channel checks
• TypeScript, localization/style checks, 163 UI tests, and 9 dev-slot tests
• Four independent read-only reviewers return no blockers (second round); small test-only follow-up reviewed locally
• GitHub CI passed for the production fix, including Windows release build
• GitHub CI on final fixture-isolation commit, including Windows release build
• Manual Windows dashboard cancellation check