Catch the 'Nothing(SVC)' placeholder serial in both fallback sites (issue #83)

The ARTIK051_DONGLE_REF firmware family reports the literal string
"Nothing(SVC)" as serialNum on every unit -- non-empty, so the existing
`if not serial` checks in config_flow.py's _probe_and_validate and
coordinator.py's _run_discovery don't catch it. Two such appliances on
one install (a fridge and a freezer, each its own dongle) then collide:
config_flow gives both the same unique_id and rejects the second as
"already configured" (bug 2), and even once that's worked around,
device_serial feeds every entity's unique_id too, so the second
appliance's entities get silently dropped with "does not generate
unique IDs" log lines (bug 4).

Add _is_placeholder_serial (duplicated in both modules rather than
imported, to avoid pulling config_flow into the runtime coordinator's
import graph or vice versa for a two-line check) and treat it the same
as an empty serial: fall back to host/port.

Type detection (bug 1) and the door sensor's field-name gap (bug 3),
also reported in this issue, are already fixed via the
claude/device-support-issue-77-freezer branch, which hit the same
ARTIK051_DONGLE_REF family from a different report -- not duplicated
here.

custom_components/localthings/config_flow.py

@@ -197,6 +197,18 @@ def _find_live_ports(host: str, ports: list[int], timeout: float) -> list[int]:
     return _order_candidates(live)
 
 
+def _is_placeholder_serial(serial: str) -> bool:
+    """True for a non-empty serialNum that isn't actually a real identity.
+
+    The ARTIK051_DONGLE_REF firmware family reports the literal string
+    'Nothing(SVC)' for every unit -- non-empty, so the plain `if not
+    serial` check here (and the equivalent one in coordinator.py's
+    `_run_discovery`) doesn't catch it, and two such units get the same
+    config-entry unique_id / entity unique_ids and collide (issue #83).
+    """
+    return serial.strip().lower().startswith('nothing')
+
+
 def _probe_and_validate(host: str, ca_cert_pem: str, ca_key_pem: str) -> dict:
     """Fetch UUID, mint leaf cert, probe each port. Returns config entry data dict."""
     import cbor2
@@ -261,7 +273,7 @@ def _probe_and_validate(host: str, ca_cert_pem: str, ca_key_pem: str) -> dict:
                 .get('/information/vs/0', {})
                 .get('x.com.samsung.da.serialNum', '')
             )
-            if not serial:
+            if not serial or _is_placeholder_serial(serial):
                 serial = f"{host}:{port}"
             one_ui_version = (
                 resources

custom_components/localthings/coordinator.py

@@ -49,6 +49,22 @@ class _NoOpDescriptor:
 _RECOVERY_RETRY_S = 600.0  # re-attempt observe mode this often while polling
 
 
+def _is_placeholder_serial(serial: str) -> bool:
+    """True for a non-empty serialNum that isn't actually a real identity.
+
+    The ARTIK051_DONGLE_REF firmware family reports the literal string
+    'Nothing(SVC)' for every unit -- non-empty, so the plain `if not
+    serial` check below doesn't catch it, and `device_serial` feeds both
+    the HA device-registry identifier and every entity's unique_id
+    (entity.py), so two such units on the same install silently collide
+    and the second one's entities get dropped (issue #83). Mirrors the
+    identical helper in config_flow.py's `_probe_and_validate` -- kept
+    separate rather than imported to avoid pulling the config-flow module
+    into the runtime coordinator's import graph for a two-line check.
+    """
+    return serial.strip().lower().startswith('nothing')
+
+
 class LocalThingsCoordinator(DataUpdateCoordinator[dict[str, Any]]):
     """Manages one Samsung appliance: session, discovery, polling."""
 
@@ -312,7 +328,7 @@ class LocalThingsCoordinator(DataUpdateCoordinator[dict[str, Any]]):
         self._unbound_hrefs = unbound
 
         serial = info.get('x.com.samsung.da.serialNum', '')
-        if not serial:
+        if not serial or _is_placeholder_serial(serial):
             serial = self._entry.data[CONF_HOST]
         self.device_serial = serial