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.

src/vst/sampler_core.cpp

@@ -151,7 +151,14 @@ void Voice::start(int note, int velocity, const SampleData& sample, int rootNote
     if (v > 127) v = 127;
     velocityGain_ = static_cast<double>(v) / 127.0;
     ratio_ = pitchRatio(note, rootNote);
-    readPos_ = 0.0;
+    // Initial read position honors the sample's start-point offset (S11). Clamp into
+    // [0, frames): a start at or past the end degrades to 0 (play from the top) rather
+    // than starting a voice already off the end. A negative start (shouldn't occur —
+    // the map clamps) is likewise pinned to 0.
+    const std::int64_t frameCount = static_cast<std::int64_t>(sample.frames.size());
+    std::int64_t start = sample.startFrame;
+    if (start < 0 || start >= frameCount) start = 0;
+    readPos_ = static_cast<double>(start);
     sample_ = &sample;
     env_.configure(adsr);
     env_.noteOn();