fix: apply dB compression to waveform thumbnail display

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.

src/bank_grid.h

@@ -163,4 +163,24 @@ enum class NavKey { Left, Right, Up, Down, Home, End };
 Selection navigate(const Selection& current, NavKey key, int cols, int itemCount,
                    bool shift);
 
+// --- Waveform display compression --------------------------------------------
+//
+// Maps a raw linear amplitude magnitude to a perceptual display fraction so
+// quiet and medium content remains visible in the thumbnail.
+//
+// The floor below which amplitude is treated as silence (display fraction 0).
+// At -60 dB, 0.001 linear magnitude maps to ~0. Tune this constant in-DAW to
+// taste — it is the only knob for the compression curve.
+constexpr float kDisplayFloorDb = -60.0f;
+
+// Maps a signed linear amplitude value in [-1, 1] (a raw envelope extreme such
+// as PeakBin::max or PeakBin::min) to a signed display fraction in [-1, 1].
+//
+// The magnitude |linear| is converted to dB, clamped to [kDisplayFloorDb, 0],
+// then normalized so kDisplayFloorDb -> 0 and 0 dB -> 1. The original sign is
+// re-applied so positive max values still map positive (draw up) and negative
+// min values still map negative (draw down). Exact-zero input returns 0.0f
+// (stays on the midline). Full-scale (|linear| == 1.0f) returns exactly ±1.0f.
+float compressAmplitudeForDisplay(float linear);
+
 } // namespace reasampler