b3be9799e0
Linear amplitude-to-pixel mapping made -20 dBFS content reach only 10% of cell height and -40 dBFS round to zero. Replace with a dB scale (floor kDisplayFloorDb = -60 dB, in bank_grid.h) so quiet content is visible. Pure helper compressAmplitudeForDisplay() lives in bank_grid; drawThumbnail() calls it. Five new unit tests cover full-scale, zero, mid-levels, floor clamping, and sign preservation.