S11: waveform view with draggable start/loop markers + zero-crossing snap

Pure waveform_view module (frame<->pixel markers, zero-crossing snap); per-zone
loop/start overrides on PerformanceZone with self-versioning zone payload; core
start-point read offset; editor waveform surface with drag machine.

tests/test_sampler_core.cpp

@@ -520,6 +520,78 @@ static void testAbsentLoopGoesSilent() {
     for (std::size_t i = 60; i < out.size(); ++i) CHECK(approx(out[i], 0.0, 1e-6));
 }
 
+// ---------------------------------------------------------------------------
+// start point (S11): the voice's initial read position is SampleData::startFrame.
+// ---------------------------------------------------------------------------
+
+static void testStartFrameOffsetsInitialRead() {
+    // A per-frame ramp (frame i holds i*0.01) so the first rendered value pinpoints the
+    // read position. startFrame = 30 -> the first output frame reads frame 30 (0.30).
+    SampleData s;
+    s.frames.resize(100);
+    for (int i = 0; i < 100; ++i) s.frames[i] = static_cast<float>(i) * 0.01f;
+    s.rootNote = 60;
+    s.startFrame = 30;
+    Keymap km = Keymap::singleSampleChromatic(std::move(s));
+    VoiceEngine eng(1, km, flatAdsr());
+    eng.noteOn(60, 127);  // unity ratio, full velocity, flat gain
+    std::vector<AudioSample> out;
+    eng.render(out, 3);
+    CHECK(approx(out[0], 0.30, 1e-4));  // starts at frame 30, not 0
+    CHECK(approx(out[1], 0.31, 1e-4));  // advances by unity ratio
+    CHECK(approx(out[2], 0.32, 1e-4));
+}
+
+static void testStartFrameZeroIsUnchanged() {
+    // startFrame default 0 is exactly the pre-S11 behavior: read begins at frame 0.
+    SampleData s;
+    s.frames.resize(20);
+    for (int i = 0; i < 20; ++i) s.frames[i] = static_cast<float>(i) * 0.05f;
+    s.rootNote = 60;  // startFrame stays 0
+    Keymap km = Keymap::singleSampleChromatic(std::move(s));
+    VoiceEngine eng(1, km, flatAdsr());
+    eng.noteOn(60, 127);
+    std::vector<AudioSample> out;
+    eng.render(out, 1);
+    CHECK(approx(out[0], 0.0, 1e-6));  // frame 0
+}
+
+static void testStartFrameOutOfRangeClampsToZero() {
+    // A start point at/past the sample end degrades to frame 0 (play from the top), never an
+    // out-of-bounds read that would start the voice already exhausted.
+    SampleData s = dcSample(10, 60);          // 10 frames of 1.0
+    s.startFrame = 10;                         // == frameCount: out of range
+    Keymap km = Keymap::singleSampleChromatic(std::move(s));
+    VoiceEngine eng(1, km, flatAdsr());
+    eng.noteOn(60, 127);
+    std::vector<AudioSample> out;
+    eng.render(out, 5);
+    // Reads from frame 0: the DC sample plays its 1.0 body rather than an immediate idle.
+    CHECK(eng.activeVoiceCount() == 1);
+    CHECK(approx(out[0], 1.0, 1e-4));
+}
+
+static void testStartFrameWithLoop() {
+    // Start point and loop compose: begin reading mid-sample, then sustain the loop region.
+    SampleData s;
+    s.frames.resize(40);
+    for (int i = 0; i < 40; ++i) s.frames[i] = static_cast<float>(i) * 0.01f;
+    for (int i = 20; i < 40; ++i) s.frames[i] = 0.5f;   // loop body
+    s.rootNote = 60;
+    s.startFrame = 10;                                   // begin at frame 10
+    s.loop.hasLoop = true;
+    s.loop.start = 20;
+    s.loop.end = 40;
+    Keymap km = Keymap::singleSampleChromatic(std::move(s));
+    VoiceEngine eng(1, km, flatAdsr());
+    eng.noteOn(60, 127);
+    std::vector<AudioSample> out;
+    eng.render(out, 200);
+    CHECK(approx(out[0], 0.10, 1e-4));       // started at frame 10
+    CHECK(eng.activeVoiceCount() == 1);      // loop sustains it
+    for (std::size_t i = 60; i < out.size(); ++i) CHECK(approx(out[i], 0.5, 1e-4));
+}
+
 // ---------------------------------------------------------------------------
 // velocity -> volume.
 // ---------------------------------------------------------------------------
@@ -580,6 +652,10 @@ int main() {
     testZeroLengthLoopGoesSilent();
     testSingleFrameLoop();
     testAbsentLoopGoesSilent();
+    testStartFrameOffsetsInitialRead();
+    testStartFrameZeroIsUnchanged();
+    testStartFrameOutOfRangeClampsToZero();
+    testStartFrameWithLoop();
     testVelocityToVolume();
     testPolyphonyMixesAdditively();