A rest may never carry `tie: true` / `tiedToNext: true`

A tie sustains a sounding pitch; a rest has no pitch to sustain, so tying a rest is a category error, not merely an unusual case.

Gould does not discuss tying rests because it is not a representable concept in common notation; Read defines a tie strictly in terms of 'two notes of the same pitch' — a rest is definitionally excluded.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.