NOTE
sync.Once
Exactly-once process-local initialization and its concurrency semantics.
This is a historical learning note and may contain outdated or incomplete understanding.
sync.Once ensures that one supplied function is executed at most once for that Once instance, while concurrent callers wait for the single invocation to finish.
It is useful for lazy initialization and one-time registration. If the function panics, that invocation is still considered the one execution, so design initialization/retry behavior intentionally. Do not copy a Once after first use.