Review findings on head 21c3d34:
- The inspector boundary wrapped the whole WorkspaceInspectorPane, so a
  content crash deleted the fixed-width column and resize divider and left
  the flex-1 fallback as a layout sibling. Move it inside the pane around
  the rendered content only; the column chrome survives.
- It also had no resetKeys, so a healthy renderer after a route change kept
  showing the previous renderer's fallback. resetKeys now tracks the
  renderer identity.
- safeString's fallback (Object.prototype.toString) could itself throw via
  a Symbol.toStringTag getter; guard it and return a constant, with a test.