startDrag set state.selection but didn't render until pointerup's onUp
ran — and onUp can throw on `e.currentTarget.classList.remove` if the
event reference is stale after pointer capture release, which leaves
the inspector stuck on 'Nothing selected.'
One-line fix: call render() right after state.selection assignment so
the inspector + halo update from pointerdown, independent of whether
onUp completes cleanly. The drag-completion render at the end of onUp
stays — when both fire it's idempotent (renders are pure functions of
state).