feat: Add 4-Cable Method example preset (#154)

presets/05_4_Cable_Method.json

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+{
+  "format_version": 2,
+  "presetId": "05_4_Cable_Method",
+  "name": "4-Cable Method (4CM)",
+  "description": "Standard 4CM routing separating pre-amp effects (dynamics/drive) from post-amp effects (time/modulation).",
+  "routing": "graph",
+  "nodes": [
+    { "id": "n0", "type": "Input", "x": 0, "y": 250 },
+    { "id": "n1", "type": "Compressor", "x": 200, "y": 250 },
+    { "id": "n2", "type": "Overdrive", "x": 400, "y": 250 },
+    { "id": "n3", "type": "Wah", "x": 600, "y": 250 },
+    { "id": "n4", "type": "AmpSimulator", "x": 800, "y": 250 },
+    { "id": "n5", "type": "Delay", "x": 1000, "y": 250 },
+    { "id": "n6", "type": "Reverb", "x": 1200, "y": 250 },
+    { "id": "n7", "type": "Output", "x": 1400, "y": 250 }
+  ],
+  "links": [
+    { "src_pin": "n0.out0", "dst_pin": "n1.in0" },
+    { "src_pin": "n1.out0", "dst_pin": "n2.in0" },
+    { "src_pin": "n2.out0", "dst_pin": "n3.in0" },
+    { "src_pin": "n3.out0", "dst_pin": "n4.in0" },
+    { "src_pin": "n4.out0", "dst_pin": "n5.in0" },
+    { "src_pin": "n5.out0", "dst_pin": "n6.in0" },
+    { "src_pin": "n6.out0", "dst_pin": "n7.in0" }
+  ]
+}

presets/PRESETS.md

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+## 05. 4-Cable Method (4CM)
+
+The 4-Cable Method is an industry-standard routing technique that takes advantage of our DAG architecture. It separates your signal chain into two distinct phases: **Pre-Amp** (Front of House) and **Post-Amp** (FX Loop). 
+
+By placing dynamics and drive pedals (Compressor, Overdrive, Wah) *before* the Amp Simulator, you drive the input stage naturally. By placing time-based effects (Delay, Reverb) *after* the Amp Simulator, you ensure your echoes and reflections remain clean and un-muddied by the amp's distortion.
+
+### Signal Flow
+
+```mermaid
+graph LR
+%% Styling
+classDef io fill:#2d3748,stroke:#4a5568,stroke-width:2px,color:#fff;
+classDef preamp fill:#9b2c2c,stroke:#f56565,stroke-width:2px,color:#fff;
+classDef amp fill:#744210,stroke:#ecc94b,stroke-width:2px,color:#fff;
+classDef fxloop fill:#2b6cb0,stroke:#63b3ed,stroke-width:2px,color:#fff;
+
+%% Nodes
+In([🎸 Input]):::io
+Out([🔊 Output]):::io
+
+subgraph Pre-Amp Effects [Pre-Amp / Front of House]
+    Comp(Compressor):::preamp
+    OD(Overdrive):::preamp
+    Wah(Wah-Wah):::preamp
+end
+
+Amp[Amp Simulator]:::amp
+
+subgraph FX Loop [Post-Amp / FX Loop]
+    Delay(Delay):::fxloop
+    Reverb(Reverb):::fxloop
+end
+
+%% Connections
+In --> Comp
+Comp --> OD
+OD --> Wah
+Wah --> Amp
+Amp --> Delay
+Delay --> Reverb
+Reverb --> Out