Remove visualization module, migrate to hedvis package

[Copilot]

The visualization code (word cloud generation) has been extracted into a separate hedvis package to reduce core dependencies and improve modularity.

Changes

• Removed modules: hed/tools/visualization/ (284 lines)
• Removed tests: tests/tools/visualization/ (202 lines) and test data
• Dependencies removed: matplotlib>=3.9.0, wordcloud==1.9.5, implicit pillow
• Dependencies updated: Added pandas version constraint <3.0.0
• Documentation: Added migration note pointing to hedvis package

Migration

# Before (removed):
from hed.tools.visualization import create_wordcloud

# After:
# pip install hedvis
from hedvis import create_wordcloud

Notes

• No internal dependencies on visualization module found
• hedvis package published and available on PyPI
• Breaking change: users importing hed.tools.visualization will need to install hedvis separately

Original prompt

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