Add delay tempo sync, rack faceplate, README, docs

- Add host-tempo-synced delay: FX screen gains a 4th SYNC row for
  rhythmic divisions (1/16 .. 1/1) that lock delay time to DAW tempo
- Embed rack faceplate background (b2.png) via juce binary data
- Add offline dev probe (rmx_probe) for GUI-less MIDI testing
- Allow overriding JUCE_ROOT when configuring the build
- Write comprehensive README describing the instrument, UI, MIDI, DSP,
  and build/install steps

Source/SynthEngine.h

@@ -11,6 +11,7 @@ struct VParams
     float lfoRate = 4.2f, lfoShape = 0, lfoDepth = 0.22f, lfoTarget = 0;
     float fxType = 1, fxAmt = 0.30f, fxTime = 0.28f;
     float vol = 0.85f, pan = 0.0f, poly = 10.f, glide = 0.f, tune = 0.f;
+    float hq = 0.0f;
 
     void update (const juce::AudioProcessorValueTreeState& apvts)
     {
@@ -28,6 +29,7 @@ struct VParams
         fxType   = get ("fxtype");   fxAmt = get ("fxamt");   fxTime = get ("fxtime");
         vol      = get ("vol");      pan = get ("pan");       poly = get ("poly");
         glide    = get ("glide");    tune = get ("tune");
+        hq       = get ("hq");
     }
 };
 
@@ -108,7 +110,6 @@ public:
     void render (const VParams& p, float* left, float* right, int numSamples);
 
     int  activeVoices() const noexcept  { return active; }
-    void getMeters (float& l, float& r) const noexcept { l = metL; r = metR; }
 
 private:
     struct Voice
@@ -123,6 +124,7 @@ private:
         int   lfoIdx = 0;
         Env amp, flt;
         float lp = 0.0f, bp = 0.0f, f1c = 0.0f;
+        float fcSm = 0.0f;
         float outSmooth = 0.0f;
     };
 
@@ -135,7 +137,6 @@ private:
     bool pedal = false;
     float lastNoteFreq = 440.0f, lastGlide = 0.0f, lastPan = 0.0f;
     int stealIdx = 0;
-    float metL = 0.0f, metR = 0.0f;
     juce::Random rng;
     juce::Array<int> sustainedNotes;