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.h

@@ -50,6 +50,12 @@ struct SampleData {
                                       // note-relative, so rate cancels for repitch)
     int rootNote = 60;                // MIDI note recorded at (plays at unity here)
     SampleLoop loop;                  // sustain loop, if any
+    // Initial read position (frame offset) a voice starts playback at — frame 0 by
+    // default, so an unset start point is exactly the pre-S11 behavior. S11 makes this
+    // an instrument-side per-zone override (the "start point" marker); S15 builds on it
+    // (both play modes carry a modifiable start). Clamped into [0, frames) at note-on:
+    // a start >= the sample length is a no-op (voice starts at 0), never out of bounds.
+    std::int64_t startFrame = 0;
 };
 
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