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/PluginProcessor.h

@@ -22,7 +22,7 @@ public:
     const juce::String getName() const override;
     bool acceptsMidi() const override           { return true; }
     bool producesMidi() const override          { return false; }
-    double getTailLengthSeconds() const override { return 3.0; }
+    double getTailLengthSeconds() const override { return prm::delaySyncMaxTime() + 1.0; }
 
     int getNumPrograms() override;
     int getCurrentProgram() override;
@@ -37,10 +37,8 @@ public:
 
     PresetBank&         getBank()            { return bank; }
     SynthEngine&        getEngine()          { return engine; }
-    int                 getPatchIndex() const noexcept { return patchIndex; }
+int                 getPatchIndex() const noexcept { return patchIndex; }
     void                selectPatch (int i);
-
-    void    getMeters (float& l, float& r) const noexcept { engine.getMeters (l, r); }
     int     getActiveVoices() const noexcept              { return engine.activeVoices(); }
     juce::uint32        getLastMidiTime() const noexcept  { return lastMidi; }
     float               getCpu() const noexcept           { return cpu; }
@@ -59,6 +57,8 @@ private:
     int lastPatchParam = 0;
     juce::uint32 lastMidi = 0;
     float cpu = 0.0f;
+    float dcL = 0.0f, dcR = 0.0f, dcPrevL = 0.0f, dcPrevR = 0.0f;
+    juce::AudioBuffer<float> osBuf;   // internal 2x oversample target (HQ mode)
 
     JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (RMX19AudioProcessor)
 };