Fix filter env: restore unipolar modulation so all env knobs are responsive

Reverts the sustain-relative modulation that made F.ENV SUS and the
cutoff knob feel dead while a note was held. The envelope now modulates
up from the base cutoff; at env amount 0 the cutoff equals the knob, and
lowering env amount darkens the held tone as expected.

Also extends the filter to 24/48 dB modes (extra TPT stages), adds the
matching combobox options, gives knobs a real blurred drop shadow, and
updates the AGENTS.md filterType choice count.

Source/DSP/Filter.h

@@ -2,7 +2,8 @@
 #include <cmath>
 #include <algorithm>
 
-enum class FilterType { LowPass12 = 0, LowPass24, BandPass, HighPass, Notch, NumTypes };
+enum class FilterType { LowPass12 = 0, LowPass24, BandPass, HighPass, Notch,
+                        BandPass24, HighPass24, Notch24, LowPass48, NumTypes };
 
 class Filter {
 public:
@@ -12,7 +13,7 @@ public:
     }
 
     void reset() {
-        for (int i = 0; i < 4; ++i)
+        for (int i = 0; i < 8; ++i)
             stage[i] = 0.0f;
         gCoeff = 0.0f;
         gTarget = 0.0f;
@@ -52,12 +53,30 @@ public:
         stage[2] = g * hp2 + bp2;
         stage[3] = g * bp2 + lp2;
 
+        // Stage 3 (fed by the stage-2 lowpass) — 36 dB
+        float hp3 = (lp2 - (k + g) * stage[4] - stage[5]) / denom;
+        float bp3 = g * hp3 + stage[4];
+        float lp3 = g * bp3 + stage[5];
+        stage[4] = g * hp3 + bp3;
+        stage[5] = g * bp3 + lp3;
+
+        // Stage 4 (fed by the stage-3 lowpass) — 48 dB
+        float hp4 = (lp3 - (k + g) * stage[6] - stage[7]) / denom;
+        float bp4 = g * hp4 + stage[6];
+        float lp4 = g * bp4 + stage[7];
+        stage[6] = g * hp4 + bp4;
+        stage[7] = g * bp4 + lp4;
+
         switch (filterType) {
             case FilterType::LowPass12:  return lp;
             case FilterType::LowPass24:  return lp2;
             case FilterType::BandPass:   return bp;
             case FilterType::HighPass:   return hp;
             case FilterType::Notch:      return input - bp;
+            case FilterType::BandPass24: return bp2;
+            case FilterType::HighPass24: return hp2;
+            case FilterType::Notch24:    return input - bp2;
+            case FilterType::LowPass48:  return lp4;
             case FilterType::NumTypes:   return input;
         }
         return lp;
@@ -66,7 +85,7 @@ public:
 private:
     static constexpr float pi = 3.14159265358979323846f;
     double sampleRate = 44100.0;
-    float stage[4] = {0.0f, 0.0f, 0.0f, 0.0f};
+    float stage[8] = {0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f};
 
     float gCoeff = 0.0f, gTarget = 0.0f;
     float kCoeff = 2.0f, kTarget = 2.0f;

Source/DSP/Voice.h

@@ -129,13 +129,12 @@ public:
             right *= noteVelocity * envSample;
 
             // Filter cutoff with env + keytrack.
-            // Modulate RELATIVE to the envelope's sustain level so the held
-            // (sustain) cutoff always equals the base cutoff knob regardless of
-            // env amount — the envelope only opens above (attack) and closes
-            // below (release) that centre. This keeps the cutoff knob audible
-            // at any env amount instead of collapsing when env amt is lowered.
-            float modCutoff = filterCutoff * std::pow(2.0f,
-                (fEnvSample - filterEnvSustain) * filterEnvAmount * 4.0f);
+            // Unipolar modulation UP from the base cutoff: the envelope (0..1)
+            // multiplied by env amount adds brightness on top of the cutoff
+            // knob, never going below it. Every env knob (attack/decay/
+            // sustain/release) shapes the response, and at env amount 0 the
+            // cutoff equals the knob.
+            float modCutoff = filterCutoff * std::pow(2.0f, fEnvSample * filterEnvAmount * 4.0f);
             float keyTrackFactor = std::pow(2.0f, (static_cast<float>(currentMidiNote) - 60.0f) / 12.0f * keyTrack);
             modCutoff *= keyTrackFactor;
 
@@ -197,7 +196,6 @@ public:
         filterResonance = res;
         filterType = ft;
         filterEnvAmount = fEnvAmt;
-        filterEnvSustain = fSus;
         keyTrack = kTrack;
         pan = p;
         drive = std::max(drv, 1.001f);
@@ -252,7 +250,6 @@ private:
     float filterResonance = 0.7f;
     FilterType filterType = FilterType::LowPass12;
     float filterEnvAmount = 0.0f;
-    float filterEnvSustain = 0.5f;
     float keyTrack = 0.5f;
     float pan = 0.0f;
     float drive = 1.5f;

Source/PluginEditor.cpp

@@ -47,9 +47,11 @@ void KnobLookAndFeel::drawRotarySlider(juce::Graphics& g, int x, int y, int widt
     auto rw = radius * 2.0f;
     auto angle = rotaryStartAngle + sliderPos * (rotaryEndAngle - rotaryStartAngle);
 
-    // Drop shadow
-    g.setColour(juce::Colour(0x40000000));
-    g.fillEllipse(rx + 2.0f, ry + 3.0f, rw, rw);
+    // Blurred drop shadow
+    juce::DropShadow knobShadow(juce::Colour(0x55000000), 10, juce::Point<int>(0, 5));
+    juce::Path shadowPath;
+    shadowPath.addEllipse(juce::Rectangle<float>(rx, ry, rw, rw));
+    knobShadow.drawForPath(g, shadowPath);
 
     // Outer bevel (dark bottom-right, light top-left)
     auto bevelPath = juce::Path();
@@ -620,7 +622,8 @@ MainContentComponent::MainContentComponent(ChromaFlockProcessor& p)
     setupParam(phaseOffsetKnob, phaseOffsetAttach, "phaseOffset", "PHASE");
 
     setupLabel(filterLabel, "FILTER");
-    setupCB(filterTypeBox, filterTypeAttach, "filterType", {"LP 12dB", "LP 24dB", "Band Pass", "High Pass", "Notch"});
+    setupCB(filterTypeBox, filterTypeAttach, "filterType", {"LP 12dB", "LP 24dB", "Band Pass", "High Pass", "Notch",
+            "Band Pass 24", "High Pass 24", "Notch 24", "LP 48dB"});
     setupParam(filterCutoffKnob, filterCutoffAttach, "filterCutoff", "CUTOFF");
     setupParam(filterResKnob, filterResAttach, "filterRes", "RES");
     setupParam(filterEnvAmtKnob, filterEnvAmtAttach, "filterEnvAmt", "ENV AMT");

Source/PluginProcessor.cpp

@@ -91,7 +91,8 @@ juce::AudioProcessorValueTreeState::ParameterLayout ChromaFlockProcessor::create
     // FILTER
     layout.add(std::make_unique<juce::AudioParameterChoice>(
         juce::ParameterID{"filterType", 1}, "Filter Type",
-        juce::StringArray{"LP 12dB", "LP 24dB", "Band Pass", "High Pass", "Notch"}, 0));
+        juce::StringArray{"LP 12dB", "LP 24dB", "Band Pass", "High Pass", "Notch",
+                          "Band Pass 24", "High Pass 24", "Notch 24", "LP 48dB"}, 0));
     layout.add(std::make_unique<juce::AudioParameterFloat>(
         juce::ParameterID{"filterCutoff", 1}, "Filter Cutoff",
         juce::NormalisableRange<float>(