sky plot: add per-satellite trajectory trails

- Store up to 1 hour of (elevation, azimuth) history per satellite/source
- Trail column in satellite table (checkbox, default off); clicking the
  column header toggles all trails on/off
- Trail rendered in GNSS color with alpha fading from 15 (oldest) to 200
  (newest), subsampled to max 120 points for render performance
- Trail column fixed at 42 px to avoid checkbox + empty-text visual split

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

gui/sky_widget.py

@@ -45,13 +45,13 @@ class SkyRenderThread(QThread):
         self._queue: Queue = Queue(maxsize=1)
         self._running      = True
 
-    def submit(self, sats: list, width: int, height: int):
+    def submit(self, sats: list, trails: dict, width: int, height: int):
         # Discard any pending (not yet rendered) frame so we always render latest
         try:
             self._queue.get_nowait()
         except Empty:
             pass
-        self._queue.put((list(sats), width, height))
+        self._queue.put((list(sats), trails, width, height))
 
     def stop(self):
         self._running = False
@@ -71,14 +71,14 @@ class SkyRenderThread(QThread):
                 continue
             if item is None:
                 break
-            sats, w, h = item
+            sats, trails, w, h = item
             if w > 0 and h > 0:
-                img = self._render(sats, w, h)
+                img = self._render(sats, trails, w, h)
                 self.frame_ready.emit(img)
 
     # ── Drawing (runs in render thread — QPainter on QImage is thread-safe) ──
 
-    def _render(self, sats: list, w: int, h: int) -> QImage:
+    def _render(self, sats: list, trails: dict, w: int, h: int) -> QImage:
         img = QImage(w, h, QImage.Format_RGB32)
 
         p   = QPainter(img)
@@ -89,6 +89,8 @@ class SkyRenderThread(QThread):
 
         self._draw_bg(p, w, h)
         self._draw_grid(p, cx, cy, max_r)
+        if trails:
+            self._draw_trails(p, cx, cy, max_r, trails)
         self._draw_satellites(p, cx, cy, max_r, sats)
         self._draw_legend(p, sats)
 
@@ -138,6 +140,27 @@ class SkyRenderThread(QThread):
         p.setPen(Qt.NoPen)
         p.drawEllipse(QPointF(cx, cy), 3, 3)
 
+    def _draw_trails(self, p: QPainter, cx, cy, max_r, trails: dict):
+        for (_, gnss_id, _), points in trails.items():
+            n = len(points)
+            if n < 2:
+                continue
+            base_color = _GNSS_QCOLORS.get(gnss_id, _C_LABEL)
+            r, g, b = base_color.red(), base_color.green(), base_color.blue()
+
+            # Subsample to at most 120 points so rendering stays fast
+            step = max(1, n // 120)
+            pts  = points[::step]
+            m    = len(pts)
+
+            for i in range(1, m):
+                # Alpha fades from 15 (oldest) to 200 (newest)
+                alpha = int(15 + 185 * i / m)
+                p.setPen(QPen(QColor(r, g, b, alpha), 1.5))
+                x1, y1 = _polar_xy(cx, cy, max_r, pts[i - 1][0], pts[i - 1][1])
+                x2, y2 = _polar_xy(cx, cy, max_r, pts[i][0],     pts[i][1])
+                p.drawLine(QPointF(x1, y1), QPointF(x2, y2))
+
     def _draw_satellites(self, p: QPainter, cx, cy, max_r, sats):
         f = QFont(); f.setPointSize(7)
         p.setFont(f)
@@ -188,8 +211,9 @@ class SkyPlotWidget(QWidget):
 
     def __init__(self):
         super().__init__()
-        self._sats:  list       = []
-        self._image: QImage     = None
+        self._sats:   list  = []
+        self._trails: dict  = {}
+        self._image:  QImage = None
 
         self.setMinimumSize(400, 400)
         self.setSizePolicy(QSizePolicy.Expanding, QSizePolicy.Expanding)
@@ -206,16 +230,17 @@ class SkyPlotWidget(QWidget):
 
     # ── Public API ────────────────────────────────────────────────────────────
 
-    def update_data(self, sats: list):
-        self._sats = sats
-        self._thread.submit(sats, self.width(), self.height())
+    def update_data(self, sats: list, trails: dict = None):
+        self._sats   = sats
+        self._trails = trails or {}
+        self._thread.submit(sats, self._trails, self.width(), self.height())
 
     # ── Qt events ─────────────────────────────────────────────────────────────
 
     def resizeEvent(self, event):
         super().resizeEvent(event)
         if self._sats:
-            self._thread.submit(self._sats, self.width(), self.height())
+            self._thread.submit(self._sats, self._trails, self.width(), self.height())
 
     def paintEvent(self, _event):
         p = QPainter(self)