Coordinate Go oracle termination across process launch

[rohenaz]

Summary

• model Go-oracle child launch, pending shutdown, running, and exited states explicitly
• queue shutdown requests until Process.run() has assigned a positive PID
• serialize graceful and forced termination for persistent and one-shot children
• preserve the original typed responseTooLarge failure while shutdown races normal exit

Why

PR #1 exposed a pre-existing Swift Foundation 6.1 launch-order race on Linux: Process.isRunning becomes true before processIdentifier is assigned. A pipe callback can therefore reach Process.terminate() during launch and trip its positive-PID precondition with SIGILL. This is a test-harness baseline fix, not BRC-100 production behavior.

Verification

• overlong response regression: 30/30 repeated passes
• focused GoOracleProtocolTests: 18/18 passed
• full swift test: 796 passed
• exact Linux CI is the runtime-specific proof