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

Makefile

@@ -17,6 +17,10 @@ CMAKE      := cmake
 BUILD_DIR_ABSPATH := $(abspath $(BUILD_DIR))
 ARTEFACTS  := $(BUILD_DIR)/RMX19_artefacts/$(CONFIG)
 
+JUCE_DIR   := $(BUILD_DIR)/JUCE
+JUCE_REPO  := https://github.com/juce-framework/JUCE.git
+JUCE_TAG   := 8.0.4
+
 VST3_SRC   := $(ARTEFACTS)/VST3/$(PLUGIN_BASE).vst3
 AU_SRC     := $(ARTEFACTS)/AU/$(PLUGIN_BASE).component
 
@@ -28,17 +32,25 @@ USER_PLUGIN_DIRS := $(HOME)/Library/Audio/Plug-Ins/VST3 $(HOME)/Library/Audio/Pl
 .EXPORT_ALL_VARIABLES:
 CMAKE_BUILD_TYPE := $(CONFIG)
 
-.PHONY: all configure build install uninstall clean info
+.PHONY: all configure build install uninstall clean info juce
 
 all: build
 
 build: configure
 	$(CMAKE) --build $(BUILD_DIR) --config $(CONFIG) --parallel
 
+juce:
+	@if [ ! -d "$(JUCE_DIR)" ]; then \
+		echo "Fetching JUCE $(JUCE_TAG)..."; \
+		git clone --branch $(JUCE_TAG) --depth 1 $(JUCE_REPO) $(JUCE_DIR); \
+	else \
+		echo "JUCE already present at $(JUCE_DIR)"; \
+	fi
+
 # Re-run cmake on every invocation: it is cheap when nothing changed and keeps
 # the footer's git branch/hash and local build timestamp fresh.
-configure:
-	$(CMAKE) -S . -B $(BUILD_DIR) -DCMAKE_BUILD_TYPE=$(CONFIG)
+configure: juce
+	$(CMAKE) -S . -B $(BUILD_DIR) -DCMAKE_BUILD_TYPE=$(CONFIG) -DJUCE_ROOT=$(JUCE_DIR)
 
 install: build
 	@codesign --force --deep --sign - "$(VST3_SRC)" >/dev/null 2>&1 || true