fix: guard spectrogram buffer against height=2

A buffer of height 2 made half-1 zero, dividing by zero in pushColumn
and writing NaN into the column. Clamp the denominator and add a
regression test for the 2-row case.

oscope-sphere/src/SpectrogramBuffer.cpp

@@ -30,22 +30,21 @@ float SpectrogramBuffer::norm01(float magnitude) {
 
 void SpectrogramBuffer::pushColumn(const std::vector<float>& magCh1,
                                    const std::vector<float>& magCh2) {
-    const int col  = writeIndex_;
-    const int half = height_ / 2;
+    const int col   = writeIndex_;
+    const int half  = height_ / 2;
+    const float denom = static_cast<float>(std::max(1, half - 1));
 
     // CH1 -> northern rows [half, height_): row half = equator (low freq),
     // row height_-1 = north pole (high freq).
     for (int r = half; r < height_; ++r) {
-        const float frac = static_cast<float>(r - half) /
-                           static_cast<float>(half - 1);
+        const float frac = static_cast<float>(r - half) / denom;
         data_[static_cast<std::size_t>(r) * width_ + col] =
             norm01(logResample(magCh1, frac));
     }
     // CH2 -> southern rows [0, half): row half-1 = equator (low freq),
     // row 0 = south pole (high freq).
     for (int r = 0; r < half; ++r) {
-        const float frac = static_cast<float>(half - 1 - r) /
-                           static_cast<float>(half - 1);
+        const float frac = static_cast<float>(half - 1 - r) / denom;
         data_[static_cast<std::size_t>(r) * width_ + col] =
             norm01(logResample(magCh2, frac));
     }

oscope-sphere/tests/test_spectrogram.cpp

@@ -1,5 +1,6 @@
 #include "check.h"
 #include "SpectrogramBuffer.h"
+#include <cmath>
 #include <vector>
 
 using oscope::SpectrogramBuffer;
@@ -25,8 +26,22 @@ static void test_high_freq_maps_to_poles() {
     CHECK(d[0 * W + 0] > d[3 * W + 0]);   // CH2: south pole > equator
 }
 
+// A 2-row buffer (half == 1) must not divide by zero / emit NaN.
+static void test_height_two_is_finite() {
+    SpectrogramBuffer buf(4, 2);
+    std::vector<float> m(1024, 0.0f);
+    m[500] = 50.0f;
+    buf.pushColumn(m, m);
+    const std::vector<float>& d = buf.data();
+    bool allFinite = true;
+    for (float v : d)
+        if (!std::isfinite(v)) allFinite = false;
+    CHECK(allFinite);
+}
+
 int main() {
     test_write_index_wraps();
     test_high_freq_maps_to_poles();
+    test_height_two_is_finite();
     REPORT();
 }