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.

tests/test_bank_grid.cpp

@@ -15,6 +15,7 @@
 
 #include "../src/bank_grid.h"
 
+#include <cmath>
 #include <cstdio>
 #include <string>
 #include <vector>
@@ -322,6 +323,52 @@ static void testNavDegenerate() {
     CHECK(selEq(navigate(Selection{{0}, 0, 0}, NavKey::Down, 0, 4, false), {1}, 1, 1));
 }
 
+// --- compressAmplitudeForDisplay ----------------------------------------------
+
+// Full scale: magnitude 1.0 must reach the full display fraction exactly.
+static void testCompressFullScale() {
+    CHECK(compressAmplitudeForDisplay(1.0f) == 1.0f);
+    CHECK(compressAmplitudeForDisplay(-1.0f) == -1.0f);
+}
+
+// Exact zero must stay on the midline (no log of zero; guards the singularity).
+static void testCompressZeroIsMidline() {
+    CHECK(compressAmplitudeForDisplay(0.0f) == 0.0f);
+}
+
+// -20 dB (0.1 linear) and -40 dB (0.01 linear) must both produce clearly visible
+// (non-zero) fractions, with -20 dB > -40 dB (monotonic), and both well above
+// the midline (arbitrary threshold of 0.15 chosen conservatively — at a -60 dB
+// floor, -20 dB normalizes to 2/3 and -40 dB to 1/3).
+static void testCompressMidValuesVisible() {
+    const float f20 = compressAmplitudeForDisplay(0.1f);   // -20 dBFS
+    const float f40 = compressAmplitudeForDisplay(0.01f);  // -40 dBFS
+    CHECK(f20 > 0.15f);    // clearly non-zero
+    CHECK(f40 > 0.15f);    // clearly non-zero
+    CHECK(f20 > f40);      // monotonic: louder -> taller bar
+}
+
+// At and below the floor (-60 dB = 0.001 linear) the result is ~0 (silence).
+// We test at exactly the floor magnitude and well below it.
+static void testCompressAtAndBelowFloor() {
+    // 0.001 == 10^(-60/20) is the floor ratio. Magnitude at or below it -> 0.
+    const float floorMag = std::pow(10.0f, kDisplayFloorDb / 20.0f);  // ~0.001
+    CHECK(compressAmplitudeForDisplay(floorMag) == 0.0f);
+    CHECK(compressAmplitudeForDisplay(floorMag * 0.5f) == 0.0f);
+    CHECK(compressAmplitudeForDisplay(0.0001f) == 0.0f);
+}
+
+// Sign is preserved: negative input produces a negative fraction of the same
+// magnitude as its positive counterpart.
+static void testCompressSignPreserved() {
+    const float pos = compressAmplitudeForDisplay(0.1f);
+    const float neg = compressAmplitudeForDisplay(-0.1f);
+    CHECK(neg < 0.0f);
+    // Magnitudes must be equal (sign-symmetric).
+    const float diff = pos + neg;  // pos - |neg|
+    CHECK(diff > -0.001f && diff < 0.001f);
+}
+
 int main() {
     testColumnsForWidth();
     testTooNarrowClampsToOneColumn();
@@ -355,6 +402,12 @@ int main() {
     testNavFromEmptyFocusesFirst();
     testNavDegenerate();
 
+    testCompressFullScale();
+    testCompressZeroIsMidline();
+    testCompressMidValuesVisible();
+    testCompressAtAndBelowFloor();
+    testCompressSignPreserved();
+
     if (g_fail == 0) std::printf("All tests passed.\n");
     return g_fail ? 1 : 0;
 }