Adds a new action that lets users place cut-off lines on plates by
clicking. The action auto-detects horizontal vs vertical axis based on
mouse proximity to plate edges, shows a dashed preview line that
follows the cursor, and adds the cut-off on left click. Pressing
Escape returns to selection mode.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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