A trill lasting multiple notes gets one tr sign plus a wavy extension line, not a repeated stamp

When a single sustained pitch is trilled across several written note values (tied together), engraving shows the `tr` sign once at the first note and a wavy line (trillo/tr~~~~~) continuing for the duration — never the `tr` glyph repeated on each tied segment.

A held trilled half note is authored (because the model has no note-spanning ornament) as two tied quarter notes, EACH carrying `ornament: 'trill'` — the render shows `tr` twice over one sustained pitch instead of once with a continuation line.

Gould, Behind Bars, ch. Ornaments — the trill extension (wavy) line convention; Read, Music Notation, gives the same rule for sustained trills.

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