Rail gets a drag handle on its edge (240-640px, width persisted like
the rest of the panel state). Escape now closes the panel as the last
step of its chain (dialog > inline > selection > panel). The dev panel
remembers its tab across reloads. Panel toggle button title notes the
Esc shortcut. Harness covers resize/persist, Esc-close and dev tab.
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Co-Authored-By: Codebuff <noreply@codebuff.com>