feat: live cursor readout (time / frequency / STFT level)

Floating tag follows the pointer over the spectrogram showing the time,
frequency, and STFT magnitude (dB) under the cursor — the standard
spectrum-analyzer probe. Suppressed while selecting/panning (which have
their own readout) and when a modal is open.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/render.h

@@ -25,6 +25,7 @@ void DrawSpectrogramGrid(Rectangle bounds, int numCellsX, int numCellsY, Color c
 void DrawLabels(Rectangle bounds);
 void DrawSelection(Rectangle bounds);
 void DrawSelectionDrag(Rectangle bounds);
+void DrawCursorReadout(Rectangle bounds);
 void DrawPlayhead(Rectangle bounds);
 
 #endif // RENDER_H