Move bus runtime into tdactor

[DanShaders]

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[github-actions]

1. 5b4f5e95 regression: valid create_child flow now hard-fails for unregistered bus types
   The new guard CHECK(bus_parents_.contains(node->type_id)); in BusRuntime.h:571 rejects buses that were never seen in register_actor.
   That breaks legitimate usage where a child bus is created only for structure/lifetime and has no actors spawning on it (example: test-runtime.cpp:367). Before this commit, such buses worked (no spawned actors, but bus could still exist).
   Suggested fix: register bus ancestry at bus creation time (start<B> and child creation path) rather than requiring prior actor registration for that exact bus type.

[github-actions]

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