fix(code-review): resolve Batch 2 open findings (AbCip, AbLegacy, Galaxy, FOCAS)

- Driver.AbCip.Contracts-001: parse 'writable' from TagConfig JSON (default true) instead of hardcoding
- Driver.AbCip.Contracts-002/-003: Dt type comment; drop dead [Display]/[Range] annotations
- Driver.AbCip.Contracts-004: dedicated AbCipEquipmentTagParser test class (+15)
- Driver.AbCip-017: document Tick severity Low-fallback on Bad severity read
- Driver.AbLegacy.Contracts-002/-003/-004: isArray-scalar remarks (+tests), MaxTagBytes/ForFamily docs
- Driver.Galaxy.Browser-003 + Driver.Galaxy.Contracts-003: extract ResolveApiKey -> GalaxySecretRef (dedup)
- Driver.Galaxy-019: cache buffered-interval only on Ok + ILogger warnings + ClassifyIntervalReply (+tests)
- Driver.FOCAS.Contracts-002: thread WriteIdempotent through DiscoverAsync (+test)

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipAlarmProjection.cs

@@ -181,6 +181,11 @@ internal sealed class AbCipAlarmProjection : IAsyncDisposable
             if (inFaultedDv.StatusCode != AbCipStatusMapper.Good) continue;
 
             var nowFaulted = ToBool(inFaultedDv.Value);
+            // severityDv.StatusCode is not checked here. When the Severity read is Bad (value null),
+            // ToInt(null) returns 0 and MapSeverity buckets it as Low. This is acceptable because
+            // InFaulted and Severity are members of the same ALMD UDT read in one batch, so a Good
+            // InFaulted almost always implies a Good Severity. The "unknown severity → Low" fallback
+            // is intentional and matches the behaviour documented on Driver.AbCip-017.
             var severity = ToInt(severityDv.Value);
 
             var wasFaulted = sub.LastInFaulted.GetValueOrDefault(nodeId, false);