v1.3: Auto makeup gain, spectrum analyzer, FIR normalization, README overhaul

- Auto makeup gain: RMS-based loudness compensation from actual FIR response
- Real-time FFT spectrum analyzer behind EQ curves
- FIR normalization fix: flat settings now produce exact 0 dB passthrough
- Brickwall limiter (0 dB ceiling) with toggle
- Drag-and-drop signal chain reordering
- Low FIR tap count warning for 512/1024
- Double-click reset on all knobs
- Comprehensive README with linear phase EQ explanation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Source/SignalChainPanel.cpp

@@ -21,7 +21,7 @@ juce::String SignalChainPanel::getStageName (InstaLPEQProcessor::ChainStage stag
     {
         case InstaLPEQProcessor::MasterGain: return "MASTER GAIN";
         case InstaLPEQProcessor::Limiter:    return "LIMITER";
-        case InstaLPEQProcessor::MakeupGain: return "MAKEUP GAIN";
+        case InstaLPEQProcessor::MakeupGain: return "AUTO GAIN";
         default: return "?";
     }
 }