Position-keyed comparison only catches "did this exact slot spot change,"
not "is this same Slot object already mounted somewhere else" -- which is
exactly the double-mount invariant Slot already promises for its elements.
A Relate<slot, inst> enforces it directly: same owner -> spurious re-emit,
ignore; different owner -> error; no owner -> bind.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
