Give same-second event writers distinct file names

Two EventsWriter instances created on the same host in one second open the same event file. Creating the second writer truncates the first writer's events.

Reproduced on Linux / Python 3.12 against upstream commit 48a5f8aee532e086bebacfd0613260874e8899f9. A deterministic regression and focused fix are prepared.

This report was prepared with AI assistance.