Add all-channel presets to channel compare

[itsDNNS]

Summary

• add All Downstream and All Upstream presets to the existing channel compare flow
• allow the compare API to load larger all-channel selections for these presets
• keep large overlays readable by disabling per-point markers when many channels are shown

Closes #206

Testing

• python -m pytest tests/test_channel_timeline.py -q
• python -m pytest tests/e2e/test_charts.py -k all_downstream -q