[emeraldsingers]

Tracks with no singer, or with a singer that is missing or not loaded, are currently silent. This adds a built-in sine renderer so such tracks play a tone that follows the notes and pitch curve.

• New SineRenderer is used automatically when a track has no usable singer. It follows the pitch curve and supports DYN and PITD.
• The sine phrases go through the existing phrase pipeline (PhraseSource / RenderPhrase). A track without a singer now builds phrases from its notes, and the fallback skips expression graph processing.
• New preference "Play sine tone when no singer is selected" in Preferences > General, on by default. When off, those tracks stay silent and the track header no longer shows SINE.
• The track header shows SINE only while the fallback is active. It is removed as soon as a real singer is assigned.
• The preference is part of the render hash, so toggling it re-renders affected phrases.