Add block customization to visual search

[CormacAltman]

Summary

• add automatic silent audio stand-ins for missing radio prompt clips so communications trials no longer crash when a sample is absent
• rework the visual search task to run timed preview → mask → search cycles with configurable intervals, randomized 16-item grids, and color-coded feedback flashes
• expose a yellow color constant to support the timeout feedback overlay
• add block-aware visual search configuration with per-block item folders, fallback captions, and distance controls plus folder-based item loading

Testing

• python -m py_compile plugins/visualsearch.py

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Codex Task