Add BobinkNode QML type and two-page demo for node monitoring

BobinkNode (QQuickItem) monitors a single OPC UA node with
automatic lifecycle: monitoring starts/stops based on item
visibility (StackView page switches, Loader, etc.).

Properties: nodeId, value (r/w), status, sourceTimestamp,
serverTimestamp. On-demand readAttribute() for metadata.
writeError signal for failed writes.

Demo restructured with StackView: connection page → two node
pages demonstrating the visibility-based monitoring lifecycle.

1 files changed, 164 insertions, 0 deletions

diff --git a/demo/NodePage.qml b/demo/NodePage.qml
new file mode 100644
index 0000000..e4862d8
--- /dev/null
+++ b/demo/NodePage.qml
@@ -0,0 +1,164 @@
+// NodePage.qml — Monitors a single OPC UA node.
+// Two instances demonstrate visibility lifecycle: switching pages
+// stops monitoring on the hidden page automatically.
+
+import QtQuick
+import QtQuick.Controls
+import QtQuick.Layouts
+import Bobink
+
+Page {
+    id: nodePage
+
+    required property StackView stackRef
+    required property int pageNumber
+    required property var logFunction
+
+    property string monitoredNodeId: ""
+    property string nodeDescription: ""
+    property bool nodeWritable: true
+    property bool metadataLoaded: false
+
+    BobinkNode {
+        id: node
+        nodeId: nodePage.monitoredNodeId
+        onAttributeRead: (name, val) => {
+            if (name === "DisplayName")
+                nodePage.nodeDescription = val.toString();
+            else if (name === "AccessLevel")
+                nodePage.nodeWritable = (val & 0x02) !== 0;
+            nodePage.metadataLoaded = true;
+        }
+        onWriteError: (message) => nodePage.logFunction("WRITE: " + message)
+    }
+
+    ColumnLayout {
+        anchors.fill: parent
+        anchors.margins: 20
+        spacing: 12
+
+        RowLayout {
+            Label {
+                text: "Node Page " + nodePage.pageNumber
+                font.bold: true
+                font.pointSize: 14
+            }
+            Item { Layout.fillWidth: true }
+            Button {
+                text: "Disconnect"
+                onClicked: BobinkClient.disconnectFromServer()
+            }
+        }
+
+        RowLayout {
+            TextField {
+                id: nodeIdField
+                Layout.fillWidth: true
+                placeholderText: "ns=2;s=Temperature"
+                enabled: !nodePage.monitoredNodeId
+            }
+            Button {
+                text: nodePage.monitoredNodeId ? "Stop" : "Monitor"
+                onClicked: {
+                    if (nodePage.monitoredNodeId) {
+                        nodePage.monitoredNodeId = "";
+                        nodePage.nodeDescription = "";
+                        nodePage.nodeWritable = true;
+                        nodePage.metadataLoaded = false;
+                    } else {
+                        nodePage.monitoredNodeId = nodeIdField.text;
+                        node.readAttribute("DisplayName");
+                        node.readAttribute("AccessLevel");
+                    }
+                }
+            }
+        }
+
+        GroupBox {
+            Layout.fillWidth: true
+            visible: nodePage.monitoredNodeId.length > 0
+            title: nodePage.nodeDescription || nodePage.monitoredNodeId
+
+            GridLayout {
+                columns: 2
+                width: parent.width
+
+                Label { text: "Value:" }
+                Label {
+                    text: node.value !== undefined ? node.value.toString() : "—"
+                    font.family: "monospace"
+                    font.bold: true
+                }
+
+                Label { text: "Status:" }
+                Label {
+                    text: node.status === BobinkNode.Good ? "Good"
+                        : node.status === BobinkNode.Uncertain ? "Uncertain"
+                        : "Bad"
+                    color: node.status === BobinkNode.Good ? "green"
+                         : node.status === BobinkNode.Uncertain ? "orange"
+                         : "red"
+                }
+
+                Label { text: "Source TS:" }
+                Label {
+                    text: node.sourceTimestamp.toLocaleString(
+                              Qt.locale(), "HH:mm:ss.zzz")
+                    font.family: "monospace"
+                }
+
+                Label { text: "Server TS:" }
+                Label {
+                    text: node.serverTimestamp.toLocaleString(
+                              Qt.locale(), "HH:mm:ss.zzz")
+                    font.family: "monospace"
+                }
+            }
+        }
+
+        RowLayout {
+            visible: nodePage.monitoredNodeId.length > 0
+
+            TextField {
+                id: writeField
+                Layout.fillWidth: true
+                enabled: nodePage.nodeWritable
+                placeholderText: nodePage.metadataLoaded && !nodePage.nodeWritable
+                                 ? "Read-only node" : "Value to write"
+            }
+            Button {
+                text: "Write"
+                enabled: nodePage.nodeWritable
+                onClicked: {
+                    node.value = writeField.text;
+                    writeField.clear();
+                }
+            }
+        }
+
+        Label {
+            visible: nodePage.monitoredNodeId.length > 0
+                     && nodePage.metadataLoaded && !nodePage.nodeWritable
+            text: "Read-only node"
+            font.italic: true
+            color: "gray"
+        }
+
+        Item { Layout.fillHeight: true }
+
+        Button {
+            Layout.fillWidth: true
+            text: nodePage.pageNumber === 1 ? "Go to Page 2" : "Back to Page 1"
+            onClicked: {
+                if (nodePage.pageNumber === 1)
+                    nodePage.stackRef.push("NodePage.qml", {
+                        stackRef: nodePage.stackRef,
+                        pageNumber: 2,
+                        logFunction: nodePage.logFunction
+                    });
+                else
+                    nodePage.stackRef.pop();
+            }
+        }
+    }
+}