fix(preserve): prime SOLA rings with real source (zero-latency, gap-free onset) + sub-sample splice alignment — clean repitch C1..C8

src/vst/sampler_core.h

@@ -466,7 +466,8 @@ public:
     // meaningful while active(). NOTE (Phase S re-scope of FA1): the unity-shift demotion to
     // Varispeed is now OPT-IN via start()'s unityVarispeedBypass — only the preview card takes
     // it; a MIDI Preserve voice keeps its shifter at every note so a chromatic line has one
-    // uniform onset (no ~25 ms step at the root).
+    // uniform onset. GA2 update: the primed shifter speaks on frame 0 at every ratio, so the
+    // demotion is purely a per-frame work-skip — onset timing is uniform either way.
     PitchEngine pitchEngine() const { return pitchEngine_; }
     // The SampleData this voice is playing (nullptr when never started). The engine's mono
     // legato path compares it against the new note's resolved sample — a same-sample takeover
@@ -474,11 +475,12 @@ public:
     const SampleData* playingSample() const { return sample_; }
 
     // Pre-SIZE this voice's Preserve pitch shifters (both channels) to `windowFrames`, OFF the
-    // audio thread (this allocates). The engine calls it once at construction so start() — which
-    // runs on the audio thread inside process() — never allocates: start() only reset()s + warm()s
-    // the already-sized rings. `windowFrames` <= 1 leaves the shifters as pass-through (Varispeed
-    // instruments pay no ring cost). Idempotent: a re-presize to the same window is a cheap no-op
-    // in the underlying vector.
+    // audio thread (this allocates; also sizes the prime scratch buffer). The engine calls it
+    // once at construction so start() — which runs on the audio thread inside process() — never
+    // allocates: start() only prime()s the already-sized rings with the first window of source
+    // (a bounded copy). `windowFrames` <= 1 leaves the shifters as pass-through (Varispeed
+    // instruments pay no ring cost). Idempotent: a re-presize to the same window is a cheap
+    // no-op in the underlying vector.
     void presizePreserveShifters(std::int64_t windowFrames);
 
     // Renders one frame's contribution, advancing the read head and envelope by one
@@ -511,6 +513,12 @@ private:
     // finishes (Gate: release complete; Trigger: play length reached) so advanceFrame frees the voice.
     double tickAmplitude();
 
+    // True when the sustain loop applies to this voice: GATE mode with a valid, non-empty loop
+    // inside the sample (S15 — Trigger one-shots never loop). The single source of truth for
+    // the wrap rule shared by the output anchor (readPos_), the Preserve feed (feedPos_), and
+    // the start()-time ring prime.
+    bool sustainLoopUsable() const;
+
     bool active_ = false;
     bool releasing_ = false;
     int note_ = 0;
@@ -533,10 +541,20 @@ private:
     // S16 pitch engine + pitch envelope. pitchEngine_ selects Varispeed (ratio bias) vs Preserve
     // (source-rate read + shifter). shiftL_/shiftR_ transpose the Preserve output per channel
     // (one read head, per-channel shift — S7 compose). pitchEnv_ rides EITHER engine.
+    //
+    // GA2 onset fix: the shifter rings are PRIMED at start() with the first window of the
+    // actual upcoming source (loop-unrolled, silence past the end) — output frame 0 is source
+    // frame `start`, no ring-fill silence, and splices always land in real history. feedPos_
+    // is the integer SOURCE frame the shifters are fed next; it runs exactly one window AHEAD
+    // of readPos_ (the wall-clock output anchor) under the same sustain-loop wrap rule.
+    // primeBuf_ is the presized scratch the prime stream is assembled into (never touched
+    // outside start()).
     PitchEngine pitchEngine_ = PitchEngine::Varispeed;
     PitchEnvelope pitchEnv_;
     PitchShifter shiftL_;
     PitchShifter shiftR_;
+    std::int64_t feedPos_ = 0;
+    std::vector<AudioSample> primeBuf_;
 
     // Takeover declick state (see kDeclickDecay above). lastOut{L,R}_ track the voice's
     // most recent rendered output (post-gain, incl. any running declick) so a takeover/steal