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.cpp

@@ -3,6 +3,7 @@
 #include "bank_grid.h"
 
 #include <algorithm>
+#include <cmath>
 
 namespace reasampler {
 
@@ -199,4 +200,28 @@ Selection navigate(const Selection& current, NavKey key, int cols, int itemCount
     return s;
 }
 
+float compressAmplitudeForDisplay(float linear) {
+    const float mag = linear < 0.0f ? -linear : linear;
+
+    // The linear magnitude at the floor threshold: 10^(kDisplayFloorDb/20).
+    // Any magnitude at or below this maps to display fraction 0.
+    // Computed once as a constant expression; std::pow is constexpr in C++20 but
+    // not C++17, so derive it via the floor definition directly at runtime — it is
+    // only called once per bin, and the branch-free math is cheap.
+    const float floorMag = std::pow(10.0f, kDisplayFloorDb / 20.0f);
+
+    if (mag <= floorMag) return 0.0f;  // below floor (and guards log10(0))
+
+    // dB in [kDisplayFloorDb, 0] for magnitude in [floorMag, 1].
+    const float db = 20.0f * std::log10(mag);
+
+    // Normalize to [0, 1]: 0 at kDisplayFloorDb, 1 at 0 dB.
+    const float fraction = (db - kDisplayFloorDb) / (0.0f - kDisplayFloorDb);
+
+    // Clamp to [0, 1] so floating-point overshoot on |linear| > 1.0 stays bounded,
+    // then re-apply the original sign.
+    const float clamped = fraction < 0.0f ? 0.0f : (fraction > 1.0f ? 1.0f : fraction);
+    return linear < 0.0f ? -clamped : clamped;
+}
+
 } // namespace reasampler