Unity 6.6: reset Extenject static state for Fast Enter Play Mode

[CharlieHess]

Summary

In a consumer project testing Unity 6.6 alpha, the remaining Fast Enter Play Mode problems clustered in Extenject. Most of them came from static contexts, caches, and pools that were only safe because domain reload used to clear them between play sessions.

Unity 6.6 is moving the other way. Fast Enter Play Mode is the default for new projects, and the existing reset hooks here were still editor-gated and tied to EditorSettings.enterPlayModeOptionsEnabled. That left the no-domain-reload path depending on behavior that was no longer guaranteed.

This branch makes those resets explicit at SubsystemRegistration and adds the missing reset coverage for static memory pools plus the generic DictionaryPool, HashSetPool, and ListPool helpers. These are the changes we used to keep an embedded-Extenject consumer project working on Unity 6000.6.0a2 with domain reload disabled.

I am not presenting this as a complete Unity 6.6 support pass for the repository's sample project. The validation here came from the downstream consumer-project migration, and I checked in UNITY_6_6_FAST_ENTER_PLAY_MODE_NOTES.md to make that provenance explicit.