Never select or delete a hidden decision placeholder with Backspace, Delete or arrows

Backspace/Delete beside a collapsed decision now join the text blocks
around it, and a node selection that lands on a hidden placeholder is
handed on to the next visible block, so a visible card after a hidden
one is what a vertical arrow selects.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>