Investigate macOS child-process shutdown timeout — no validated fix

[Copilot]

Description

Investigated the macOS timeout in Shutdown_UsesSharedGracefulBudgetAndEscalates. No repository changes were made; the evidence does not establish a repair.

• Evidence: Confirmed the exact failure twice. Earlier and latest comparable builds each passed all eight theory cases.
• Blockers: macOS reproduction requires approved execution budgets and workflow-dispatch access.
• Outcome: Inconclusive. Root cause remains unconfirmed; this is not a fix PR.

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
    • If yes, did you have an API Review for it?
      • Yes
      • No
    • Did you add <remarks /> and <code /> elements on your triple slash comments?
      • Yes
      • No
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
    • If yes, have you done a threat model and had a security review?
      • Yes
      • No
  • No