Fix Signal name reset and orphaned references on device reconfigure

Apply & Reconnect rebuilt a device's whole Signal list from hardware
defaults on every call, silently discarding user/profile-set names,
units, and colors even when pins hadn't changed. Add
merge_signal_metadata() and call it from ArduinoDevice.remap_pins()
and SerialDevice.reconfigure() to reapply the same fields
ProfileManager already treats as user overrides, keyed by signal_id.

Also close a related gap: removing a pin that a Channel/Plot/Control
still referenced left it silently dangling, and SignalProcessor never
pruned its latest-value cache, so a derived Channel kept computing off
a permanently frozen stale reading. Add SignalProcessor.
on_device_reconfigured() to purge that cache, reference_registry.
find_references() for a console breadcrumb, and live "missing source"
indicators on Plot traces and Channel blocks reusing ControlWidget's
existing stale/QSS pattern. Also fixes ControlWidget._write() still
enqueuing writes to a channel that no longer exists.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

1 files changed, 16 insertions, 3 deletions

diff --git a/ui/strip_chart.py b/ui/strip_chart.py
index bdffb91..b295a8c 100644
--- a/ui/strip_chart.py
+++ b/ui/strip_chart.py
@@ -176,9 +176,13 @@ class StripChartWidget(QWidget):
 
             for tr in spec.traces:
                 if not tr.visible: continue
-                pen   = pg.mkPen(color=tr.color, width=tr.width,
-                                  style=_STYLES.get(tr.style, Qt.PenStyle.SolidLine))
-                curve = plot.plot([], [], pen=pen, name=tr.label or tr.channel_id)
+                missing = self._signal_missing(tr.device_id, tr.channel_id)
+                pen   = pg.mkPen(color="#6b7280" if missing else tr.color, width=tr.width,
+                                  style=Qt.PenStyle.DashLine if missing
+                                        else _STYLES.get(tr.style, Qt.PenStyle.SolidLine))
+                label = tr.label or tr.channel_id
+                curve = plot.plot([], [], pen=pen,
+                                   name=f"{label} (missing)" if missing else label)
                 key   = (tr.device_id, tr.channel_id)
                 self._curves.setdefault(key, []).append(
                     {"curve": curve, "plot": plot, "spec": spec, "pane": spec})
@@ -203,6 +207,15 @@ class StripChartWidget(QWidget):
         for e in self._curves.get(key, []):
             e["curve"].setVisible(enabled)
 
+    def _signal_missing(self, device_id: str, channel_id: str) -> bool:
+        """True if this trace's (device_id, channel_id) no longer resolves —
+        e.g. Apply & Reconnect dropped the pin — so it can be flagged instead
+        of silently freezing with no explanation."""
+        if device_id == "derived":
+            return not any(dc.channel_id == channel_id for dc in self.processor.get_derived())
+        dev = self.registry.get_instance(device_id)
+        return dev is None or dev.get_signal(channel_id) is None
+
     def _lookup_channel_unit(self, device_id: str, channel_id: str) -> str:
         if device_id == "derived":
             for dc in self.processor.get_derived():