fix: use combine(..., axis="t") in optogenetic demo notebook

[hinderling]

Summary

• Replace removed RTMSequence + operator with combine(..., axis="t") in the two multi-phase cells of experiments/02_demo_sim_optogenetic/demo_sim_optogenetic.ipynb.
• Update surrounding markdown to document the new API.
• Add combine to the import line.

The other 11 notebooks that mention RTMSequence already use combine() — this was the last holdout.

Test plan

• Notebook runs end-to-end with the virtual-microscope extra installed (uv sync --extra virtual-microscope --python 3.12); outputs regenerated.