feat(map): add peak highlights and live metrics to PNG export

- Draw rings around hottest and coldest sensors on export.
- Extend export_image to support second line for live metrics.
- Format detailed sensor stats in export footer.
- Add test coverage for footer rows, overlays, and stats.

src/pygui/backend/visualization/wafer_map_item.py

@@ -27,6 +27,7 @@ from PySide6.QtGui import (
 from PySide6.QtQml import QmlElement
 from PySide6.QtQuick import QQuickPaintedItem
 
+from pygui.backend.models.frame_stats import compute_stats
 from pygui.backend.visualization.rbf_heatmap import (
     ellipse_alpha,
     interpolate_field,
@@ -36,6 +37,16 @@ from pygui.backend.wafer.zwafer_models import Sensor
 
 log = logging.getLogger(__name__)
 
+
+def readout_text(values: list[float]) -> str:
+    """Full READOUT stats line for the export footer; '' when no data."""
+    if not values:
+        return ""
+    s = compute_stats(values)
+    return (f"Sensors: {len(values)}   Min: {s.min:.2f}°C (#{s.min_index + 1})   "
+            f"Max: {s.max:.2f}°C (#{s.max_index + 1})   Diff: {s.diff:.2f}   "
+            f"Avg: {s.avg:.2f}   σ: {s.sigma:.2f}   3σ: {s.three_sigma:.2f}")
+
 QML_IMPORT_NAME = "ISC.Wafer"
 QML_IMPORT_MAJOR_VERSION = 1
 
@@ -301,11 +312,13 @@ class WaferMapItem(QQuickPaintedItem):
     #   chosen output dir, plus an optional summary CSV of (file, min/max/mean).
     #   No new rendering code needed — see docs/pending/alpha-release-polish-plan.md §6.1.
     @Slot(str, result=bool)
-    def export_image(self, file_path: str) -> bool:
+    @Slot(str, str, result=bool)
+    def export_image(self, file_path: str, extra: str = "") -> bool:
         """Export the current wafer map rendering to a PNG file
 
         Args:
-            file_pat: Absolute path for the output PNG.
+            file_path: Absolute path for the output PNG.
+            extra: Optional second footer line (live stream metrics).
 
         Returns:
         True on success, False on failure
@@ -321,33 +334,47 @@ class WaferMapItem(QQuickPaintedItem):
             w, h = int(self.width()), int(self.height())
             if w <= 0 or h <= 0:
                 return False
-            metrics = self._metrics_text()
-            footer_h = 28 if metrics else 0
+            lines = [ln for ln in (readout_text(self._values), extra) if ln]
+            footer_h = 28 * len(lines)
             img = QImage(w, h + footer_h, QImage.Format.Format_ARGB32)
             img.fill(0)  # transparent background
             painter = QPainter(img)
             self.paint(painter)
-            if metrics:
-                footer = QRectF(0, h, w, footer_h)
-                painter.fillRect(footer, QColor("#101014"))
+            self._paint_extremes(painter)
+            if lines:
+                painter.fillRect(QRectF(0, h, w, footer_h), QColor("#101014"))
                 painter.setPen(QColor("#CBD5E1"))
-                painter.drawText(footer, Qt.AlignmentFlag.AlignCenter, metrics)
+                for i, line in enumerate(lines):
+                    painter.drawText(QRectF(0, h + 28 * i, w, 28),
+                                     Qt.AlignmentFlag.AlignCenter, line)
             painter.end()
             return bool(img.save(file_path))
         except Exception as e:
             log.error("Export failed: %s", e)
             return False
 
-    def _metrics_text(self) -> str:
-        """One-line metric readout for the export footer; '' when no data."""
-        if not self._values:
-            return ""
-        vs = self._values
-        return (f"Sensors: {len(vs)}   Min: {min(vs):.2f}°C   "
-                f"Max: {max(vs):.2f}°C   Avg: {sum(vs) / len(vs):.2f}°C")
 
     # ── internal ─────────────────────────────────────────────────────────
 
+    def _paint_extremes(self, painter: QPainter) -> None:
+        """Ring the hottest (high color) and coldest (low color) markers.
+
+        Export-only overlay — paint() itself stays untouched so the live
+        view is unchanged.
+        """
+        if not self._values or not self._markers:
+            return
+        s = compute_stats(self._values)
+        ring_r = self._marker_r + 5
+        painter.setBrush(Qt.BrushStyle.NoBrush)
+        for idx, color in ((s.max_index, self._high_color),
+                           (s.min_index, self._low_color)):
+            if idx not in self._markers:
+                continue
+            px, py = self._markers[idx]
+            painter.setPen(QPen(color, 2))
+            painter.drawEllipse(px - ring_r, py - ring_r, 2 * ring_r, 2 * ring_r)
+
     def _rebuild_thickness(self) -> None:
         """Interpolate thickness data into a gray/orange heatmap QImage."""
         if not self._sensors or not self._thickness_data: