P6.4 follow-up: diffuser config schema additions

The P6.4 diffuser refactor reads headerPressure, localAtmPressure,
waterDensity, and zoneVolume out of the config. validateSchema strips
unknown keys, so without these definitions the values fell out of the
config object before specificClass could read them. Added with
sensible defaults that match the pre-refactor inline constants.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/configs/diffuser.json

@@ -106,6 +106,34 @@
         "type": "number",
         "description": "Alpha factor used for oxygen transfer correction."
       }
+    },
+    "headerPressure": {
+      "default": 0,
+      "rules": {
+        "type": "number",
+        "description": "Header gauge pressure above atmospheric (mbar)."
+      }
+    },
+    "localAtmPressure": {
+      "default": 1013.25,
+      "rules": {
+        "type": "number",
+        "description": "Local atmospheric pressure (mbar)."
+      }
+    },
+    "waterDensity": {
+      "default": 997,
+      "rules": {
+        "type": "number",
+        "description": "Water density used in head-pressure calculation (kg/m3)."
+      }
+    },
+    "zoneVolume": {
+      "default": 0,
+      "rules": {
+        "type": "number",
+        "description": "Aeration zone volume used to convert oxygen output to reactor OTR (m3)."
+      }
     }
   }
 }