fix: recognize all-same-hex-digit placeholder serials (#189)

_is_placeholder_serial only caught the ARTIK051_DONGLE_REF family's
'Nothing(SVC)' sentinel. The DA_WM_A51_20_COMMON (ARTIK051) laundry
board family reports a different one instead -- every character the
same repeated hex digit -- which passed through as a real, non-unique
serial. Two different physical units (a washer and a dryer) both
reporting the literal serialNum 'FFFFFFFFFFFFFFF' collided on the
config-entry unique_id, so the second device's config flow aborted as
already configured. Widen the check (both the config_flow.py and
coordinator.py copies) to also catch that sentinel shape.

custom_components/localthings/config_flow.py

@@ -215,8 +215,23 @@ def _is_placeholder_serial(serial: str) -> bool:
     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).
+
+    Issue #189: the DA_WM_A51_20_COMMON (ARTIK051) laundry board family
+    reports a flash-unset sentinel instead -- every character the same
+    repeated hex digit (a washer and a dryer, two different physical
+    units, both reported the literal serialNum 'FFFFFFFFFFFFFFF') -- which
+    the 'nothing' check above doesn't catch either, so the second unit's
+    config flow aborted as already configured.
     """
-    return serial.strip().lower().startswith('nothing')
+    s = serial.strip()
+    if s.lower().startswith('nothing'):
+        return True
+    upper = s.upper()
+    return (
+        len(upper) >= 8
+        and len(set(upper)) == 1
+        and upper[0] in '0123456789ABCDEF'
+    )
 
 
 def _probe_and_validate(host: str, ca_cert_pem: str, ca_key_pem: str) -> dict:

custom_components/localthings/coordinator.py

@@ -92,12 +92,30 @@ def _is_placeholder_serial(serial: str) -> bool:
     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.
+    and the second one's entities get dropped (issue #83).
+
+    Issue #189: the DA_WM_A51_20_COMMON (ARTIK051) laundry board family
+    reports a flash-unset sentinel instead -- every character the same
+    repeated hex digit (a washer and a dryer, two different physical
+    units, both reported the literal serialNum 'FFFFFFFFFFFFFFF') -- which
+    the 'nothing' check above doesn't catch either, so two such units
+    collided on the config-entry unique_id and the second couldn't be
+    added at all.
+
+    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')
+    s = serial.strip()
+    if s.lower().startswith('nothing'):
+        return True
+    upper = s.upper()
+    return (
+        len(upper) >= 8
+        and len(set(upper)) == 1
+        and upper[0] in '0123456789ABCDEF'
+    )
 
 
 class LocalThingsCoordinator(DataUpdateCoordinator[dict[str, Any]]):