feat: run the per-voice filter between the pitch and amp stages, with its own deck

Params ride the one parameter set; payload v8 -> v9, off by default.
Deck composition moves to a pure deck_groups module in pitch -> filter -> amp order.

src/core/instrument/engine/voice.cpp

@@ -92,6 +92,24 @@ void Voice::start(int note, int velocity, const SampleData& sample, bool declick
     pitchEnv_.configure(p.pitchEnv);
     pitchEnv_.noteOn();
 
+    // Filter: fresh integrators per note (prepare() preserves state on purpose, so a note-on
+    // is the one place that must clear it). Velocity maps through the curve once here, off the
+    // per-frame path, exactly as the amp's velocityGain_ does.
+    filterOn_ = p.filter.enabled;
+    if (filterOn_) {
+        filterSettings_ = p.filter.settings;
+        filterCutoffNorm_ = static_cast<double>(p.filter.settings.cutoffNorm);
+        filterModAmount_ = p.filter.modAmount;
+        filterKeyTrack_ = p.filter.keyTrack;
+        filterVelOffset_ =
+            p.filter.velAmount * p.filter.velocityCurve.eval(static_cast<double>(velocity));
+        filterRate_ = static_cast<double>(sample.sampleRate);
+        filterEnv_.configure(p.filter.env);
+        filterEnv_.noteOn();
+        filter_.reset();
+        updateFilterCutoffBase(note);
+    }
+
     // Prime the already-sized per-channel shifters with the first window of the actual
     // upcoming source stream (loop-unrolled under the sustain-loop wrap rule; silence past
     // the sample end, since that silence is the true stream there). The tap parks on source
@@ -162,6 +180,9 @@ void Voice::retune(int note) {
     if (!active_ || sample_ == nullptr) return;
     note_ = note;
     baseRatio_ = keyTrackedRatio(note, sample_->rootNote, sample_->keyTrack);
+    // Filter key-tracking follows the pitch: it is a function of the note, so a slide moves it
+    // too. The velocity offset deliberately stays the first note's, matching velocityGain_.
+    if (filterOn_) updateFilterCutoffBase(note);
 }
 
 void Voice::release() {
@@ -169,6 +190,7 @@ void Voice::release() {
     if (playMode_ == PlayMode::Trigger) return;  // Trigger ignores note-off, plays through
     releasing_ = true;
     env_.noteOff();
+    filterEnv_.noteOff();
 }
 
 }  // namespace reasampler