feat: add synthetic demo signal source

Pure-C++ continuous two-channel generator used when no scope is
connected: CH1 frequency sweep, CH2 steady tone plus noise. Adds the
run_tests.sh runner covering all three pure-C++ units.

oscope-sphere/src/DemoSignal.cpp

@@ -0,0 +1,41 @@
+#include "DemoSignal.h"
+#include <algorithm>
+#include <cmath>
+
+namespace oscope {
+
+namespace {
+constexpr double kTwoPi = 6.283185307179586;
+}
+
+DemoSignal::DemoSignal(float sampleRateHz)
+    : sr_(sampleRateHz > 1.0f ? sampleRateHz : 48000.0f) {}
+
+float DemoSignal::frand() {
+    rng_ = rng_ * 1664525u + 1013904223u;
+    return (static_cast<float>(rng_ >> 8) / 8388608.0f) - 1.0f;  // [-1,1)
+}
+
+void DemoSignal::next(std::vector<float>& ch1, std::vector<float>& ch2,
+                      std::size_t n) {
+    ch1.resize(n);
+    ch2.resize(n);
+    for (std::size_t i = 0; i < n; ++i) {
+        // CH1: sweep 80 Hz .. 2000 Hz, sweep period ~6 s.
+        sweep_ += 1.0 / sr_;
+        const double sweepHz =
+            80.0 + 960.0 * (1.0 + std::sin(kTwoPi * sweep_ / 6.0));
+        phase1_ += kTwoPi * sweepHz / sr_;
+        ch1[i] = 0.85f * static_cast<float>(std::sin(phase1_));
+
+        // CH2: steady 440 Hz tone + light noise.
+        phase2_ += kTwoPi * 440.0 / sr_;
+        const float v =
+            0.7f * static_cast<float>(std::sin(phase2_)) + 0.15f * frand();
+        ch2[i] = std::clamp(v, -1.0f, 1.0f);
+    }
+    if (phase1_ > kTwoPi * 1e6) phase1_ -= kTwoPi * 1e6;
+    if (phase2_ > kTwoPi * 1e6) phase2_ -= kTwoPi * 1e6;
+}
+
+} // namespace oscope

oscope-sphere/src/DemoSignal.h

@@ -0,0 +1,28 @@
+#pragma once
+#include <cstdint>
+#include <vector>
+
+namespace oscope {
+
+// Pure-C++ synthetic two-channel signal for when no scope is connected.
+// CH1: slow frequency sweep. CH2: steady 440 Hz tone plus light noise.
+// Stateful: phase advances across calls so the output stays continuous.
+class DemoSignal {
+public:
+    explicit DemoSignal(float sampleRateHz);
+
+    // Fills ch1 and ch2 with n freshly generated samples each.
+    void next(std::vector<float>& ch1, std::vector<float>& ch2,
+              std::size_t n);
+
+private:
+    float frand();   // cheap LCG noise in [-1, 1)
+
+    float    sr_;
+    double   phase1_ = 0.0;
+    double   phase2_ = 0.0;
+    double   sweep_  = 0.0;
+    uint32_t rng_    = 0x9E3779B9u;
+};
+
+} // namespace oscope

oscope-sphere/tests/run_tests.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+# Compiles and runs every pure-C++ unit test. No openFrameworks needed.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+CXX="${CXX:-clang++}"
+FLAGS=(-std=c++17 -O0 -g -Isrc)
+
+echo "== analysis =="
+"$CXX" "${FLAGS[@]}" tests/test_analysis.cpp src/FFT.cpp src/AudioAnalyzer.cpp \
+  -o /tmp/osph-test-analysis
+/tmp/osph-test-analysis
+
+echo "== spectrogram =="
+"$CXX" "${FLAGS[@]}" tests/test_spectrogram.cpp src/SpectrogramBuffer.cpp \
+  -o /tmp/osph-test-spectro
+/tmp/osph-test-spectro
+
+echo "== demo =="
+"$CXX" "${FLAGS[@]}" tests/test_demo.cpp src/DemoSignal.cpp \
+  -o /tmp/osph-test-demo
+/tmp/osph-test-demo
+
+echo "ALL TESTS PASSED"

oscope-sphere/tests/test_demo.cpp

@@ -0,0 +1,41 @@
+#include "check.h"
+#include "DemoSignal.h"
+#include <cmath>
+#include <vector>
+
+using oscope::DemoSignal;
+
+// Output has the requested length, stays in [-1,1], and the two
+// channels differ.
+static void test_shape_and_bounds() {
+    DemoSignal demo(48000.0f);
+    std::vector<float> a, b;
+    demo.next(a, b, 256);
+    CHECK(a.size() == 256);
+    CHECK(b.size() == 256);
+    bool inRange = true, differ = false;
+    for (std::size_t i = 0; i < a.size(); ++i) {
+        if (std::fabs(a[i]) > 1.0f || std::fabs(b[i]) > 1.0f) inRange = false;
+        if (std::fabs(a[i] - b[i]) > 1e-4f) differ = true;
+    }
+    CHECK(inRange);
+    CHECK(differ);
+}
+
+// Phase is continuous across calls: a second block differs from the first.
+static void test_phase_advances() {
+    DemoSignal demo(48000.0f);
+    std::vector<float> a1, b1, a2, b2;
+    demo.next(a1, b1, 64);
+    demo.next(a2, b2, 64);
+    bool same = true;
+    for (std::size_t i = 0; i < 64; ++i)
+        if (std::fabs(a1[i] - a2[i]) > 1e-4f) same = false;
+    CHECK(!same);
+}
+
+int main() {
+    test_shape_and_bounds();
+    test_phase_advances();
+    REPORT();
+}