R1.5 GetTagExtendedPropertiesAsync (GetTepByNm) + R1.6 closed (no op)

Ship tag extended-property reads over the 2020 WCF aa/Retr/GetTepByNm op:
HistorianClient.GetTagExtendedPropertiesAsync(tag) -> name/value pairs.

String-handle op reached with the Open2 storage-session GUID formatted
uppercase (same format that unlocked GETRP/GETHI/ExeC). Routed via the
name-based native path (GetTagExtendedPropertiesByName, server-fetch flag),
not the index-based TagQuery path.

Evidence-backed findings from the capture:
- GetTepByNm (and GetTgByNm) succeed with the uppercase handle -- further
  validates the resolved string-handle wall.
- QTB (StartTagQuery) does NOT punch through: captured uppercase, it still
  fails server-side (CMdServer::StartActiveTagnamesQuery over the
  aahMetadataServer pipe) -- a metadata-server blocker, not handle format.
- R1.6 (localized properties) has NO distinct op (only error-message/UI-text
  localization in the managed client); collapses into R1.5. Closed, not throwing.

Wire format (golden-pinned, synthetic bytes -- no dev tag names committed):
- request tagNames = uint count + per-name(uint charCount + UTF-16)
- response = uint tagCount + per-tag(marker + compact-ASCII name +
  uint propCount + per-prop(marker + compact-ASCII name + 0x43 VT_BSTR value)
  + trailer); sequence-paged.

Adds: HistorianTagExtendedProperty model, HistorianTagExtendedPropertyProtocol
(codec), HistorianWcfTagExtendedPropertyClient (orchestration), dialect +
public API; golden WcfTagExtendedPropertyProtocolTests (4) + gated live test
(HISTORIAN_TEP_TAG). Tooling: Capture-TagExtendedProperties.ps1,
decode-tag-properties-capture.py, harness tag-extended-properties scenario.
Docs: wcf-tag-extended-properties.md; roadmap R1.5 DONE / R1.6 collapsed;
wall doc + memory updated with the QTB-server-side nuance. 228 tests green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01B6mcaT2PjRFKcogzp9UkfC

src/AVEVA.Historian.Client/HistorianClient.cs

@@ -166,6 +166,19 @@ public sealed class HistorianClient : IAsyncDisposable
         return _protocol.GetRuntimeParameterAsync(name, cancellationToken);
     }
 
+    /// <summary>
+    /// Reads the extended (user-defined) properties attached to a tag via the 2020 WCF
+    /// <c>GetTepByNm</c> op. Returns the property name/value pairs (empty when the tag has none).
+    /// String-valued properties only (the evidence-backed surface); other value variants throw
+    /// <see cref="ProtocolEvidenceMissingException"/>. See
+    /// <c>HistorianTagExtendedPropertyProtocol</c>.
+    /// </summary>
+    public Task<IReadOnlyList<HistorianTagExtendedProperty>> GetTagExtendedPropertiesAsync(string tag, CancellationToken cancellationToken = default)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(tag);
+        return _protocol.GetTagExtendedPropertiesAsync(tag, cancellationToken);
+    }
+
     /// <summary>
     /// Creates or updates the named tag in the Historian Runtime database via
     /// <c>EnsureTags2</c>. Currently only <see cref="HistorianDataType.Float"/> is