fix(runner): protect teardown from late TERM

[Chemaclass]

🤔 Background

Related #1376

A timeout can interrupt teardown after its sleeping child exits, leaving cleanup unfinished.

💡 Changes

• Protect cleanup from a late TERM after the watchdog marks the timeout, while keeping its two-second KILL bound.
• Keep cancellation bounded before and after timeout, and run teardown once.
• Cover delayed signals, slow and hanging cleanup, and output capture in sequential, parallel and strict runs.
• Verify source and standalone runners on macOS Bash 3.2 and GNU Bash 3.0, including repeated runs, plus the full local gate.