feat: permissive unit check for user-defined measurement types + measurement digital-mode schema

MeasurementContainer.isUnitCompatible now short-circuits to accept any unit
when the measurement type is not in the built-in measureMap. Known types
(pressure, flow, power, temperature, volume, length, mass, energy) still
validate strictly. This unblocks user-defined types in the measurement
node's new digital/MQTT mode — e.g. 'humidity' with unit '%', 'co2' with
'ppm' — without forcing those units into the convert-module unit system.

measurement.json schema: add 'mode.current' (analog | digital) and
'channels' (array) so the validator stops stripping them from the runtime
config. Ignored in analog mode.

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

src/configs/measurement.json

@@ -411,6 +411,34 @@
             }
         }
     },
+    "mode": {
+        "current": {
+            "default": "analog",
+            "rules": {
+                "type": "enum",
+                "values": [
+                    {
+                        "value": "analog",
+                        "description": "Single-scalar input mode (classic 4-20mA / PLC style). msg.payload is a number; the node runs one offset/scaling/smoothing/outlier pipeline and emits one MeasurementContainer slot."
+                    },
+                    {
+                        "value": "digital",
+                        "description": "Multi-channel input mode (MQTT / IoT JSON style). msg.payload is an object keyed by channel names declared under config.channels; the node routes each key through its own pipeline and emits N slots from one input message."
+                    }
+                ],
+                "description": "Selects how incoming msg.payload is interpreted."
+            }
+        }
+    },
+    "channels": {
+        "default": [],
+        "rules": {
+            "type": "array",
+            "itemType": "object",
+            "minLength": 0,
+            "description": "Channel map used in digital mode. Each entry is a self-contained pipeline definition: {key, type, position, unit, scaling?, smoothing?, outlierDetection?, distance?}. Ignored in analog mode."
+        }
+    },
     "outlierDetection": {
         "enabled": {
             "default": false,