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/PluginEditor.h

@@ -2,6 +2,7 @@
 #include <juce_gui_basics/juce_gui_basics.h>
 #include "LcdPanel.h"
 #include "PluginProcessor.h"
+#include "BinaryData.h"
 
 class RackEditor : public juce::AudioProcessorEditor, private juce::Timer
 {
@@ -42,11 +43,17 @@ private:
     NavButton* left = nullptr;  NavButton* right = nullptr;
     OkButton* enterB = nullptr; OkButton* backB = nullptr;
 
+    juce::ComboBox scaleBox;
     LcdPanel display;
 
     juce::Colour face;
+    juce::Image backgroundImage;
     float powerPulse = 0.0f;
+    float uiScale = 1.5f;
 
+    static constexpr int baseW = 656, baseH = 248;
+
+    void setScale (float s);
     void timerCallback() override;
 
     JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (RackEditor)