feat: add rolling spectrogram column buffer

Pure-C++ store that log-resamples both channels' FFT magnitudes into
scrolling columns, CH1 north and CH2 south, high frequencies toward
the poles. No GL dependency, fully unit-tested.

oscope-sphere/tests/test_spectrogram.cpp

@@ -0,0 +1,32 @@
+#include "check.h"
+#include "SpectrogramBuffer.h"
+#include <vector>
+
+using oscope::SpectrogramBuffer;
+
+// writeIndex advances modulo width.
+static void test_write_index_wraps() {
+    SpectrogramBuffer buf(8, 4);
+    std::vector<float> z(1024, 0.0f);
+    for (int i = 0; i < 10; ++i) buf.pushColumn(z, z);
+    CHECK(buf.writeIndex() == 2);                 // 10 % 8
+    CHECK(static_cast<int>(buf.data().size()) == 8 * 4);
+}
+
+// High-frequency energy must land near the poles for both channels.
+static void test_high_freq_maps_to_poles() {
+    SpectrogramBuffer buf(4, 8);   // H=8 -> CH1 rows 4..7, CH2 rows 0..3
+    std::vector<float> hi(1024, 0.0f);
+    hi[1023] = 100.0f;             // energy at the top FFT bin
+    buf.pushColumn(hi, hi);        // both channels: high-frequency content
+    const std::vector<float>& d = buf.data();
+    const int W = buf.width();
+    CHECK(d[7 * W + 0] > d[4 * W + 0]);   // CH1: north pole > equator
+    CHECK(d[0 * W + 0] > d[3 * W + 0]);   // CH2: south pole > equator
+}
+
+int main() {
+    test_write_index_wraps();
+    test_high_freq_maps_to_poles();
+    REPORT();
+}