Fix concurrent container disposal

[Aaronontheweb]

Summary

• make ContainerNode disposal atomic and idempotent
• prevent a second teardown from completing an already-disposed R3 subject
• add a deterministic concurrent-disposal regression test

Incident evidence

Netclaw native smoke failed with ObjectDisposedException in R3 Subject.OnCompleted through ContainerNode.Dispose and KeyedDynamicLayoutNode.DisposeRetiredChildren. The regression test produces the same R3 stack on unpatched dev and passes with this change.

Validation

• dotnet build Termina.slnx -c Release --no-restore
• dotnet test Termina.slnx -c Release --no-build --no-restore: 1,713 passed
• Slopwatch on both changed files: 0 issues
• git diff --check