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.

tests/test_sample_map.cpp

@@ -610,6 +610,57 @@ static void testResolvePlayConvertsWallClockAtTheRate() {
     CHECK(at96.trigger.fadeInFrames == 441);    // still unconverted
 }
 
+static void testResolvePlayCarriesTheFilterAndResolvesOnlyItsEnvelope() {
+    // The filter's control positions are already rate-free, so only its envelope crosses the
+    // seconds->frames boundary. A converted norm would be a bug in the other direction: the
+    // same preset must sound identical at 48k and 96k.
+    PlaySeconds st;
+    st.filter.enabled = true;
+    st.filter.settings.cutoffNorm = 0.25f;
+    st.filter.settings.resonanceNorm = 0.75f;
+    st.filter.settings.morphNorm = 0.5f;
+    st.filter.settings.driveNorm = 0.125f;
+    st.filter.settings.morphLaw = reasampler::instrument::engine::filter::MorphLaw::HighNotchLow;
+    st.filter.modAmount = -0.5;
+    st.filter.velAmount = 0.25;
+    st.filter.keyTrack = 1.25;
+    st.filter.env.attackSeconds = 0.01;
+    st.filter.env.holdSeconds = 0.02;
+    st.filter.env.decaySeconds = 0.03;
+    st.filter.env.sustainLevel = 0.4;
+    st.filter.env.releaseSeconds = 0.05;
+
+    const PlayParams at48 = resolvePlay(st, 48000);
+    CHECK(at48.filter.enabled);
+    CHECK(at48.filter.settings.cutoffNorm == 0.25f);
+    CHECK(at48.filter.settings.resonanceNorm == 0.75f);
+    CHECK(at48.filter.settings.morphNorm == 0.5f);
+    CHECK(at48.filter.settings.driveNorm == 0.125f);
+    CHECK(at48.filter.settings.morphLaw == reasampler::instrument::engine::filter::MorphLaw::HighNotchLow);
+    CHECK(at48.filter.modAmount == -0.5);
+    CHECK(at48.filter.velAmount == 0.25);
+    CHECK(at48.filter.keyTrack == 1.25);
+    CHECK(at48.filter.env.attackFrames == 480);
+    CHECK(at48.filter.env.holdFrames == 960);
+    CHECK(at48.filter.env.decayFrames == 1440);
+    CHECK(at48.filter.env.sustainLevel == 0.4);   // a level, not a time
+    CHECK(at48.filter.env.releaseFrames == 2400);
+
+    const PlayParams at96 = resolvePlay(st, 96000);
+    CHECK(at96.filter.env.attackFrames == 960);
+    CHECK(at96.filter.env.releaseFrames == 4800);
+    CHECK(at96.filter.settings.cutoffNorm == 0.25f);  // rate-free: unchanged
+
+    // Off by default, and the default envelope is a flat unity so a disengaged filter has
+    // nothing to modulate with either.
+    const PlayParams bare = resolvePlay(PlaySeconds{}, 48000);
+    CHECK(!bare.filter.enabled);
+    CHECK(bare.filter.modAmount == 0.0);
+    CHECK(bare.filter.velAmount == 0.0);
+    CHECK(bare.filter.keyTrack == 0.0);
+    CHECK(bare.filter.env.sustainLevel == 1.0);
+}
+
 static void testResolvePlayRoundsAndFloorsNegatives() {
     PlaySeconds st;
     st.adsr.attackSeconds = 0.0001;   // 4.41 frames at 44.1k -> rounds to 4
@@ -845,6 +896,7 @@ int main() {
     testRetainRefsFiltersToPlayedSet();
     testLegacyLiftDecision();
     testResolvePlayConvertsWallClockAtTheRate();
+    testResolvePlayCarriesTheFilterAndResolvesOnlyItsEnvelope();
     testResolvePlayRoundsAndFloorsNegatives();
     testResolveCaptureUsesIntrinsicsWhenNoOverride();
     testResolveCaptureOverridesBeatIntrinsics();