feat(map): add batch export and adjust layout alignment

- Implement synchronous offscreen wafer map PNG rendering and CSV summary.
- Add "Batch Export" button to file browser sidebar.
- Adjust trend pane bottom spacer to align with About button when visible.
- Fix type check errors and add pytest coverage for batch export.

src/pygui/ISC/Tabs/WaferMapTab.qml

@@ -10,15 +10,20 @@ Item {
     id: root
     anchors.fill: parent
 
-    // Wire streamController.trendData (Signal(str) carrying JSON list of floats)
-    // into the trend chart's data property. The slot parseJsonToData() is defined
-    // on the Python TrendChartItem; we use it so malformed payloads are logged
-    // there instead of crashing the QML binding.
+    // Wire streamController.trendDelta (Signal(str) carrying one new
+    // [elapsed_s, value] pair per live frame) into the trend chart, which
+    // accumulates its own bounded 60s window. trendReset clears it when a
+    // new stream starts. Parsing happens on the Python TrendChartItem side
+    // so malformed payloads are logged there instead of crashing the QML
+    // binding.
     property string loadErrorMessage: ""
     Connections {
         target: streamController
-        function onTrendData(avgsJson) {
-            trendChart.setDataFromJson(avgsJson);
+        function onTrendDelta(deltaJson) {
+            trendChart.appendDelta(deltaJson);
+        }
+        function onTrendReset() {
+            trendChart.clearTrend();
         }
         function onLoadedFileChanged() {
             root.loadErrorMessage = "";
@@ -145,6 +150,8 @@ Item {
                     id: waferView
                     Layout.fillWidth: true
                     Layout.fillHeight: true
+                    Layout.preferredHeight: 400
+                    Layout.minimumHeight: 280
                     blend: readoutPanel.heatmapBlend
                     showLabels: readoutPanel.showLabels
                     showThickness: readoutPanel.showThickness
@@ -152,23 +159,31 @@ Item {
                 Rectangle {
                     id: trendPane
                     Layout.fillWidth: true
-                    implicitHeight: 160
+                    Layout.fillHeight: true
+                    Layout.preferredHeight: 220
+                    Layout.minimumHeight: 160
                     visible: trendChart.hasData && streamController.mode === "live"
                     color: Theme.cardBackground
                     border.color: Theme.cardBorder
                     border.width: 1
                     radius: Theme.radiusMd
 
-                    TrendChartItem {
-                        id: trendChart
+                    ColumnLayout {
                         anchors.fill: parent
                         anchors.margins: 8
+                        spacing: 10
+
+                        TrendChartItem {
+                            id: trendChart
+                            Layout.fillWidth: true
+                            Layout.fillHeight: true
+                        }
                     }
                 }
 
                 Item {
                     Layout.fillWidth: true
-                    implicitHeight: 16
+                    implicitHeight: trendPane.visible ? 52 : 16
                 }
                 TransportBar {
                     Layout.fillWidth: true

src/pygui/ISC/Tabs/components/StreamControlPanel.qml

@@ -275,8 +275,15 @@ Rectangle {
                 Layout.fillWidth: true
                 implicitHeight: 28
                 text: "Export"
+                // Nothing to export until a file is loaded or a stream has played
+                enabled: streamController.sensorValues.length > 0
+                opacity: enabled ? 1 : 0.4
 
-                onClicked: exportDialog.open()
+                onClicked: {
+                    // Default into <saveDataDir>/Export (created on demand)
+                    exportDialog.selectedFile = "file://" + deviceController.exportDir() + "/wafer_map.png"
+                    exportDialog.open()
+                }
 
                 background: Rectangle {
                     radius: Theme.radiusSm
@@ -299,6 +306,7 @@ Rectangle {
                 id: exportDialog
                 title: "Export Wafer Map"
                 fileMode: FileDialog.SaveFile
+                defaultSuffix: "png"
                 nameFilters: ["PNG files (*.png)"]
                 onAccepted: {
                     var path = String(selectedFile).replace(/^file:\/\//, "");