feat(mqtt): last-value cache backing IReadable (per-ref no-data)

LastValueCache bridges MQTT's subscribe-first push model to the OPC UA
server's polled IReadable.ReadAsync: the subscription path calls Update()
per RawPath, the read path calls Read(). Read never throws; an unseen
RawPath returns BadWaitingForInitialData (0x80320000) rather than an
exception or null, so a batch covering many references degrades per-ref
instead of failing wholesale.

Deviates from the plan's GoodNoData snippet: GoodNoData is reserved
repo-wide for "the historian window held no samples" (NullHistorianDataSource,
OtOpcUaNodeManager HistoryRead paths); BadWaitingForInitialData is the
established convention for "no live value observed yet" (CalculationDriver,
VirtualTagEngine, FOCAS, AddressSpaceApplier). Keyed by RawPath per the v3
driver-reference identity, not a topic/JSON-path-derived key.

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt/LastValueCache.cs

@@ -0,0 +1,63 @@
+using System.Collections.Concurrent;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Mqtt;
+
+/// <summary>
+///     Thread-safe last-observed-value store backing <see cref="IReadable"/> for MQTT/Sparkplug B.
+///     MQTT is a subscribe-first protocol — the driver holds one live broker connection and values
+///     arrive pushed, not polled. But the OPC UA server still issues polled <c>IReadable.ReadAsync</c>
+///     batches, so this cache is the bridge: the subscription path (message handler) calls
+///     <see cref="Update"/> with the newest observed value per reference, and the read path serves
+///     the most recent one via <see cref="Read"/>.
+/// </summary>
+/// <remarks>
+///     Keyed by <c>RawPath</c> — the v3 driver-reference identity (see
+///     <c>EquipmentTagRefResolver&lt;TDef&gt;</c> and <see cref="MqttTagDefinition.Name"/>,
+///     which <b>is</b> the RawPath) — never a topic/JSON-path-derived key. The parameter is
+///     therefore named <c>rawPath</c> throughout rather than "topic" or "key" to keep that identity
+///     fact visible at every call site.
+/// </remarks>
+public sealed class LastValueCache
+{
+    // OPC UA BadWaitingForInitialData (0x80320000) — the repo-wide convention for "no value has
+    // been observed yet" (mirrored by CalculationDriver before its first evaluation,
+    // VirtualTagEngine for a freshly-materialised node, and OtOpcUaNodeManager /
+    // AddressSpaceApplier for a just-deployed variable). MQTT is subscribe-first, so an unseen
+    // RawPath is in exactly that state until its first publish arrives. Deliberately NOT
+    // GoodNoData — that code is reserved (see NullHistorianDataSource, OtOpcUaNodeManager
+    // HistoryRead paths) for "the historian window held no samples", a different question from
+    // "has this live reference ever been observed".
+    private const uint BadWaitingForInitialData = 0x80320000u;
+
+    private readonly ConcurrentDictionary<string, DataValueSnapshot> _values = new();
+
+    /// <summary>
+    ///     Record the newest observed value for <paramref name="rawPath"/>. Called from the MQTT
+    ///     subscription/message-handling path. A <c>null</c> or empty <paramref name="rawPath"/>
+    ///     is a no-op — never throws.
+    /// </summary>
+    /// <param name="rawPath">The RawPath identifying the tag.</param>
+    /// <param name="snapshot">The newest observed value/quality/timestamps for that tag.</param>
+    public void Update(string rawPath, DataValueSnapshot snapshot)
+    {
+        if (string.IsNullOrEmpty(rawPath)) return;
+        _values[rawPath] = snapshot;
+    }
+
+    /// <summary>
+    ///     Read the last observed value for <paramref name="rawPath"/>. Never throws: a
+    ///     <c>null</c>/empty <paramref name="rawPath"/> or a RawPath never observed both return a
+    ///     <see cref="BadWaitingForInitialData"/> snapshot rather than an exception, so a batch read
+    ///     covering many references degrades per-reference instead of failing the whole call.
+    /// </summary>
+    /// <param name="rawPath">The RawPath identifying the tag.</param>
+    /// <returns>The last observed snapshot, or a BadWaitingForInitialData snapshot if none has been observed.</returns>
+    public DataValueSnapshot Read(string rawPath)
+    {
+        if (!string.IsNullOrEmpty(rawPath) && _values.TryGetValue(rawPath, out var snapshot))
+            return snapshot;
+
+        return new DataValueSnapshot(null, BadWaitingForInitialData, null, DateTime.UtcNow);
+    }
+}