Reap completed exec host state in the Linux runtime helper

[rogerneal]

Summary

• ExitMonitor.track now drops callback/task entries after the exit path finishes, so completed work does not retain host state forever.
• RuntimeService reaps per-exec processes / waiters on exit, failed start, and container cleanup. Late wait still works via a compact completedExits cache when no waiter was present yet.
• Failed createProcess registration and failed startExecProcess also reap instead of leaving orphaned entries.

Fixes #2057

Test plan

Tested

• swift test --filter ExitMonitorTests (3/3 passed)
  • releases entries after successful exit
  • releases entries after WaitHandler failure
  • 200 sequential track/exit cycles still allow re-registration
• Existing MultiWriterTests still compile against RuntimeService changes

Not tested

• Full long-lived container-runtime-linux malloc growth repro with thousands of container machine run ... /usr/bin/true (needs installing this build into a running daemon session and measuring helper allocations)
• Detached-exec path under allocation profiling