Respect write_to_disk for embeddings and hparams

SummaryWriter(write_to_disk=False) still creates directories and files when add_embedding or add_hparams is called. This breaks disabled logging in read-only runs.

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.