Block the final user on its cached resource's `__aexit__()` and
raise regular cleanup errors at that user's ctx boundary.
Deats,
- serialize user registration and teardown under each cache-key lock
- keep queued entrants on the same lock through resource replacement
- preserve `Cancelled`, `KeyboardInterrupt`, and `SystemExit` flow
- cover successful, failing, cancelled, and re-entry teardown paths
Prompt-IO: ai/prompt-io/opencode/20260804T030309Z_65bf9df5_prompt_io.md
(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))
Gud Boi