loop: crossfade the Gate sustain seam, and unshadow the loop handles that made loop points look gone

src/core/instrument/engine/voice.cpp

@@ -66,6 +66,10 @@ void Voice::start(int note, int velocity, const SampleData& sample, bool declick
     readPos_ = static_cast<double>(start);
     startFrame_ = start;  // the span-offset origin: readPos - startFrame
 
+    // The one fold of the stored span + crossfade into what the read path wraps on.
+    loop_ = instrument::engine::loop::resolveLoop(sample.loop, sample.loopCrossfadeFrames,
+                                                  frameCount, playMode_ == PlayMode::Gate);
+
     // Amplitude envelope: Gate = AHDSR (all five fields read from play.adsr, resolved to
     // frames from stored seconds at load time); Trigger = the staged AHD over the % play span.
     const std::int64_t postStart = frameCount - start;  // >= 1 (start clamped < frameCount)
@@ -162,9 +166,7 @@ void Voice::start(int note, int velocity, const SampleData& sample, bool declick
     // no per-frame shifter cost.
     if (pitchEngine_ == PitchEngine::Preserve && shiftL_.configured()) {
         const std::int64_t w = shiftL_.window();
-        const bool loopWrap = sustainLoopUsable();
-        const SampleLoop& loop = sample.loop;
-        const std::int64_t loopLen = loopWrap ? (loop.end - loop.start) : 0;
+        const bool loopWrap = loop_.active;
         const bool stereoSample = sample.channelCount() == 2 && shiftR_.configured();
         // The prime may only carry playable source. The per-frame feed stops at feedBound
         // (playEnd_ for a bounded Trigger span, the sample end for Gate) and freezes the
@@ -189,12 +191,18 @@ void Voice::start(int note, int velocity, const SampleData& sample, bool declick
             std::int64_t q = start;
             for (std::int64_t i = 0; i < primeCount; ++i) {
                 if (loopWrap) {
-                    while (q >= loop.end) q -= loopLen;
+                    while (q >= loop_.end) q -= loop_.length;
                 }
                 // q < frameCount holds by construction on the non-loop path (primeCount is
-                // bounded); the guard stays as a belt for the loop-wrap walk.
+                // bounded); the guard stays as a belt for the loop-wrap walk. The prime runs
+                // the SAME crossfade the per-frame feed does — a ring primed with an un-faded
+                // seam would put the click back one window into the note.
                 primeBuf_[static_cast<std::size_t>(i)] =
-                    (q < frameCount) ? pcmCh[static_cast<std::size_t>(q)] : 0.0f;
+                    (q < frameCount)
+                        ? crossfadedSource(pcmCh, q,
+                                           instrument::engine::loop::crossfadeWeight(
+                                               loop_, static_cast<double>(q)))
+                        : 0.0f;
                 ++q;
             }
             (ch == 0 ? shiftL_ : shiftR_).prime(primeBuf_.data(), primeCount);