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/ui/deck_groups.h

@@ -0,0 +1,82 @@
+// deck_groups.h — WHICH groups the Sample face's knob deck carries and in what order, plus
+// the control-id space they are built from. Pure data: knob_deck lays out whatever descriptors
+// it is handed, and this module decides what those descriptors are, so the deck's signal-flow
+// ordering is provable without a host.
+
+#pragma once
+
+#include <vector>
+
+#include "core/instrument/engine/play_params.h"  // PlayMode (the AMP group's Gate/Trigger face)
+#include "core/instrument/ui/knob_deck.h"        // DeckGroupDesc
+
+namespace reasampler::instrument::ui {
+
+// Deck control ids. Opaque to knob_deck, resolved by the shell's hit-test and value binding.
+// Runtime-only — nothing persists them, so the ordering here is free to change.
+enum class DeckParam {
+    kPlayMode = 0,   // Gate | Trigger toggle
+    kPitchEngine,    // Varispeed | Preserve toggle
+    kAttack,         // AHDSR attack   (Gate)  / —
+    kHold,           // AHDSR hold     (Gate)
+    kDecay,          // AHDSR decay    (Gate)
+    kSustain,        // AHDSR sustain  (Gate)
+    kRelease,        // AHDSR release  (Gate)
+    kTrigLength,     // Trigger %-length
+    kTrigFadeIn,     // Trigger fade-in
+    kTrigFadeOut,    // Trigger fade-out
+    kPitchEnvEnable, // AD pitch envelope on|off
+    kPitchEnvAttack, // AD pitch attack
+    kPitchEnvDecay,  // AD pitch decay
+    kPitchEnvDepth,  // AD pitch depth in +/- semitones
+    kKeyTrack,       // key-tracking 0..200% (lives on InstrumentParams, not PlaySeconds)
+    // Filter. The four control positions map through filter_params' own laws; the three
+    // depths are bipolar and centred at zero.
+    kFilterEnable,   // filter on|off caption toggle
+    kFilterMorph,    // morph position: high-pass .. low-pass
+    kFilterCutoff,   // cutoff, log across the audio band
+    kFilterQ,        // resonance
+    kFilterDrive,    // in-loop drive depth
+    kFilterModAmt,   // filter envelope -> cutoff, +/-100%
+    kFilterVel,      // velocity -> cutoff, +/-100%
+    kFilterKeyTrack, // note -> cutoff, 0..200%
+    kFilterLaw,      // morph law row toggle: HP-BP-LP | HP-notch-LP
+    kFilterEnvAttack,
+    kFilterEnvHold,
+    kFilterEnvDecay,
+    kFilterEnvSustain,
+    kFilterEnvRelease,
+    // Deck-only controls: processor-side per-instance params — routed to the processor
+    // setters, never through the parameter set.
+    kVoiceCount,     // polyphony bound (1..32) — a stepped knob in the VOICE group
+    kVoiceMode,      // Poly | Mono caption toggle (VOICE group)
+    kMonoTrigger,    // Retrig | Legato row toggle (VOICE group; live only in Mono)
+    kMasterGain,     // post-mixer master gain knob (-inf..+24 dB taper, MASTER group)
+    kCount
+};
+
+// Deck group ids. Unscoped so the shell's caption switch reads against the plain `id` int
+// knob_deck carries.
+enum DeckGroupId {
+    kGroupPitch = 0,
+    kGroupPitchEnv,
+    kGroupFilter,
+    kGroupFilterEnv,
+    kGroupAmpEnv,
+    kGroupVoice,
+    kGroupMaster,
+};
+
+// The deck's groups, left to right, in SIGNAL-FLOW order: pitch -> filter -> amp, then the
+// two instance-wide groups. `playMode` picks the AMP group's face; its width is
+// mode-independent (Trigger leaves two blank cells) so a mode flip never reflows the
+// neighbouring groups.
+std::vector<DeckGroupDesc> sampleDeckGroups(PlayMode playMode);
+
+// The deck's BIPOLAR knob law: 0.5 of the knob's travel is zero depth, the ends are -1 and
+// +1. Exact inverses, and exact at the centre detent (0.5 -> 0 -> 0.5), so a knob parked at
+// centre can never persist a hair of modulation. Out-of-range norm clamps to the endpoints.
+double deckBipolarFromNorm(double norm);
+double deckNormFromBipolar(double value);
+
+}  // namespace reasampler::instrument::ui