S-VIEW-6: per-zone keyTrack scalar (0-200%, default 100%) with pure ratio math in both repitch engines

keyTrackedRatio is bit-identical to pitchRatio at 100%; keyTrack threads through PerformanceZone->ResolvedZone->KeyZone into baseRatio_, so Varispeed and Preserve both inherit it. Zones payload bumped to v6; older blobs lift to 1.0.

src/vst/sampler_core.h

@@ -194,6 +194,11 @@ struct KeyZone {
     int lowNote = 0;
     int highNote = 127;
     int rootNote = 60;        // repitch reference for this zone
+    // S-VIEW-6 key-tracking scalar: how far keyboard pitch tracks the root. 1.0 (100%) is
+    // standard 12-tone-ET (default; bit-identical to pre-S-VIEW-6); 0.0 = no tracking (every
+    // key plays root pitch); 2.0 = double-rate tracking. Scales the (note-root) semitone offset
+    // in the repitch math (keyTrackedRatio); rides BOTH engines via the voice's baseRatio_.
+    double keyTrack = 1.0;
     std::size_t sampleIndex = 0;  // index into Keymap::samples
 };
 
@@ -227,6 +232,18 @@ struct Keymap {
 // one octave down -> 0.5. Pure equal-temperament; no reference-frequency needed.
 double pitchRatio(int note, int rootNote);
 
+// The key-tracked pitch ratio (S-VIEW-6): 2^(((note - rootNote) * keyTrack) / 12). The
+// keyTrack scalar scales the semitone offset before the ET conversion, so it governs how
+// far playback pitch tracks the keyboard around the root:
+//   keyTrack == 1.0 -> standard 12-tone-ET (BIT-IDENTICAL to pitchRatio(note, rootNote) —
+//                      (note-root)*1.0 is exact in IEEE-754, feeding the same std::pow call).
+//   keyTrack == 0.0 -> no tracking: every key plays the root pitch (ratio 1.0 for all notes).
+//   keyTrack == 2.0 -> double-rate tracking: each key is twice as far from the root in pitch.
+// At the root note the offset is 0 regardless of keyTrack, so the root always plays at unity.
+// Pure; both repitch engines (Varispeed read-rate, Preserve shift-amount) derive from it via
+// the voice's baseRatio_.
+double keyTrackedRatio(int note, int rootNote, double keyTrack);
+
 // ---------------------------------------------------------------------------
 // AHDSR amplitude envelope (S15 grows the S3 ADSR with a HOLD stage). Sample-based
 // (times in frames), linear segments. A gate: noteOn() enters Attack; noteOff() enters
@@ -351,7 +368,10 @@ public:
     // thread inside process(). The warm silence pass settles the OLA taps before the first
     // output frame (no cold-start click). Byte-identical to the pre-S15 engine when sample.play
     // is default (Gate + Varispeed + no pitch env).
-    void start(int note, int velocity, const SampleData& sample, int rootNote);
+    // `keyTrack` (S-VIEW-6) scales the (note-root) semitone offset feeding the repitch ratio;
+    // 1.0 (the default) is standard 12-tone-ET, bit-identical to the pre-S-VIEW-6 baseRatio_.
+    void start(int note, int velocity, const SampleData& sample, int rootNote,
+               double keyTrack = 1.0);
 
     // Gate off — begins the amplitude release. In GATE mode this enters the AHDSR release; in
     // TRIGGER mode it is a NO-OP (Trigger ignores note-off and plays through to its play length).