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>

2 files changed, 48 insertions, 1 deletions

diff --git a/core/reference_registry.py b/core/reference_registry.py
index b6fcd0f..7ab9649 100644
--- a/core/reference_registry.py
+++ b/core/reference_registry.py
@@ -20,7 +20,7 @@ hold such a reference and rewrites it. It expects a MainWindow instance
 reachable from.
 """
 
-from typing import Tuple
+from typing import List, Tuple
 
 Ref = Tuple[str, str]
 
@@ -72,3 +72,33 @@ def propagate_rename(app, old_ref: Ref, new_ref: Ref) -> None:
             win.refresh_derived()
         if hasattr(win, "refresh_devices"):
             win.refresh_devices()
+
+
+def find_references(app, ref: Ref) -> List[str]:
+    """Reverse lookup: human-readable names of everything currently holding
+    a (device_id, channel_id) reference — same 5 subsystems propagate_rename()
+    walks, but collecting instead of rewriting. Used to flag a signal_id an
+    Apply & Reconnect just dropped (still referenced elsewhere = orphaned),
+    without blocking the reconnect itself — see devices_window/channels_window/
+    control_panel "missing source" indicators, which do the live per-widget
+    check; this is only for a one-line summary breadcrumb."""
+    found: List[str] = []
+
+    for dc in app.processor.get_derived():
+        if ref in dc.sources:
+            found.append(f"Channel '{dc.name}'")
+
+    chart = getattr(app, "_chart", None)
+    if chart is not None and chart._cfg is not None:
+        for pane in chart._cfg.panes:
+            for tr in pane.traces:
+                if (tr.device_id, tr.channel_id) == ref:
+                    found.append(f"Plot '{pane.title}' trace '{tr.label or tr.channel_id}'")
+
+    ctrl = getattr(app, "_ctrl", None)
+    if ctrl is not None:
+        for spec in ctrl.get_specs():
+            if (spec.device_id, spec.channel_id) == ref:
+                found.append(f"Control '{spec.title}'")
+
+    return found
diff --git a/core/signal_processor.py b/core/signal_processor.py
index da615b6..c79ea87 100644
--- a/core/signal_processor.py
+++ b/core/signal_processor.py
@@ -492,6 +492,23 @@ class SignalProcessor(QObject):
     def all_virtual_channel_ids(self) -> List[str]:
         return list(self._derived_bufs.keys())
 
+    def on_device_reconfigured(self, device_id: str, current_signal_ids: set) -> List[str]:
+        """Drop cached state for any signal_id a device's Apply & Reconnect
+        just removed — otherwise _latest keeps a stale frozen (timestamp,
+        value) forever (it's only ever written to, never pruned elsewhere),
+        and a derived Channel sourced from that id keeps silently computing
+        off that frozen value instead of skipping like a genuinely-missing
+        source would. Mirrors AcquisitionEngine.on_device_reconfigured().
+        Returns the removed signal_ids (those with a cached entry keyed to
+        this device that's no longer in current_signal_ids), so the caller
+        can flag any still-dangling references without recomputing the diff."""
+        removed = []
+        with self._lock:
+            for key in [k for k in self._latest if k[0] == device_id and k[1] not in current_signal_ids]:
+                self._latest.pop(key, None)
+                removed.append(key[1])
+        return removed
+
     def clear_history(self):
         """Clear derived channel buffers and latest-value cache."""
         with self._lock: