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/play_params.h

@@ -9,6 +9,7 @@
 #include <vector>
 
 #include "core/audio/peaks.h"
+#include "core/instrument/engine/filter/voice_filter.h"
 #include "core/instrument/engine/velocity_curve.h"
 
 namespace reasampler {
@@ -90,15 +91,38 @@ struct PitchEnvParams {
     double       peakSemitones = 0.0;  // signed depth at the peak
 };
 
+// Per-voice resonant filter, off by default (enabled=false -> the render path skips it
+// entirely -> bit-identical to the un-filtered engine). Holds the filter module's OWN
+// normalized control positions verbatim rather than a parallel set, so no control range is
+// re-derived here; `filter_params.h` owns every law that maps them to Hz/Q/depth.
+//
+// The three modulation depths all land in that same normalized cutoff domain and sum before a
+// single clamp: `modAmount` scales the per-frame filter envelope, `velAmount` scales the
+// note-on velocity through `velocityCurve`, and `keyTrack` moves cutoff by octaves per octave
+// above the root. All three are zero/neutral by default.
+struct FilterParams {
+    bool enabled = false;
+    instrument::engine::filter::FilterSettings settings;
+    double modAmount = 0.0;   // bipolar [-1,+1], envelope -> cutoff
+    double velAmount = 0.0;   // bipolar [-1,+1], velocity -> cutoff
+    double keyTrack  = 0.0;   // octaves of cutoff per octave of (note - root)
+    AdsrParams env;           // the same staged AHDSR the amp runs; frames
+    // Shapes velocity before velAmount scales it. Linear rather than the amp's flat() default
+    // because a flat curve under a depth control would make every velocity the same offset;
+    // the no-op at rest is velAmount == 0, not the curve.
+    VelocityCurve velocityCurve = VelocityCurve::linear();
+};
+
 // Bundle a voice reads at start(). Defaults reproduce the bare engine (Gate, hold-0 AHDSR,
-// Varispeed, pitch envelope off) — core regression tests rely on this; the Preserve product
-// default is layered on at (de)serialization, see kDefaultPitchEngine.
+// Varispeed, pitch envelope off, filter off) — core regression tests rely on this; the
+// Preserve product default is layered on at (de)serialization, see kDefaultPitchEngine.
 struct PlayParams {
     PlayMode       playMode = PlayMode::Gate;
     AdsrParams     adsr;
     TriggerParams  trigger;
     PitchEngine    pitchEngine = PitchEngine::Varispeed;
     PitchEnvParams pitchEnv;
+    FilterParams   filter;
 };
 
 // [start, end) frames, half-open. A zero-length loop (start == end) is the "no sustain loop"