feat(presets): seed default overdrive/delay/reverb/chorus presets

Add Bank 2 'FX Bench' with 4 dedicated factory presets:
- Overdrive Bench: noise_gate + overdrive + volume
- Delay Bench: noise_gate + eq + delay + volume
- Reverb Bench: noise_gate + eq + reverb + volume
- Chorus Bench: noise_gate + eq + chorus + volume

Each preset showcases a single core effect with well-tuned default
parameters derived from the pipeline DSP schemas.

All 46 existing tests pass.

presets/factory/bank_2/preset_0.json

@@ -0,0 +1,49 @@
+{
+  "name": "Overdrive Bench",
+  "bank": 2,
+  "program": 0,
+  "master_volume": 0.8,
+  "routing_mode": "mono",
+  "routing_breakpoint": 7,
+  "tuner_enabled": false,
+  "chain": [
+    {
+      "fx_type": "noise_gate",
+      "enabled": true,
+      "bypass": false,
+      "params": {
+        "threshold": 0.01
+      },
+      "nam_model_path": "",
+      "ir_file_path": ""
+    },
+    {
+      "fx_type": "overdrive",
+      "enabled": true,
+      "bypass": false,
+      "params": {
+        "drive": 0.55,
+        "tone": 0.5,
+        "level": 0.75,
+        "gain": 0.6,
+        "bass": 0.5,
+        "mid": 0.5,
+        "treble": 0.5,
+        "presence": 0.5
+      },
+      "nam_model_path": "",
+      "ir_file_path": ""
+    },
+    {
+      "fx_type": "volume",
+      "enabled": true,
+      "bypass": false,
+      "params": {
+        "level": 0.85
+      },
+      "nam_model_path": "",
+      "ir_file_path": ""
+    }
+  ],
+  "midi_mappings": {}
+}