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/map/sample_map.h

@@ -160,6 +160,21 @@ struct PitchEnvSeconds {
     double peakSemitones = 0.0;    // signed depth at the peak
 };
 
+// The stored mirror of the engine's FilterParams (play_params.h, which owns what each field
+// MEANS). Only the envelope differs between the two: the control positions and depths are
+// rate-free already, so this block is a seconds/frames split of one field, not of the whole
+// struct. The env default is a flat unity, so `enabled` is the only thing standing between a
+// loaded blob and the pre-filter sound.
+struct FilterSeconds {
+    bool enabled = false;
+    engine::filter::FilterSettings settings;
+    double modAmount = 0.0;
+    double velAmount = 0.0;
+    double keyTrack  = 0.0;
+    AdsrSeconds env{0.0, 0.0, 0.0, 1.0, 0.0};
+    VelocityCurve velocityCurve = VelocityCurve::linear();
+};
+
 // The stored play bundle: wall-clock times in SECONDS, source-timeline quantities in
 // frames/fractions (TriggerParams). Instrument-owned, serialized, editor-facing — distinct
 // from the engine-facing PlayParams (frames).
@@ -169,6 +184,7 @@ struct PlaySeconds {
     TriggerParams   trigger;                              // Trigger: %-length + fades (source frames)
     PitchEngine     pitchEngine = kDefaultPitchEngine;    // product default: Preserve
     PitchEnvSeconds pitchEnv;                             // AD pitch modulation (seconds), off by default
+    FilterSeconds   filter;                               // per-voice filter, off by default
 };
 
 // Resolve a stored seconds bundle to the engine's frame-domain PlayParams against a live