feat(mtconnect): current/sample parse + sequence-gap detection (Task 7)

Adds the /current and /sample legs to MTConnectAgentClient:

- MTConnectStreamsParser: hand-rolled System.Xml.Linq parse of an
  MTConnectStreams document into MTConnectStreamsResult. Observations are
  every element child of <Samples>/<Events>/<Condition>, keyed by
  dataItemId (the element NAME is the data item's type in PascalCase, so
  a fixed name set would drop observations). A CONDITION's value is its
  element name, not its text - <Normal/> is empty - and <Unavailable/>
  normalizes onto the same UNAVAILABLE sentinel a Sample/Event carries as
  text, so Task 8's no-comms mapping sees one token. Timestamps are
  normalized to DateTimeKind.Utc explicitly.
- IsSequenceGap(requestedFrom, chunk) => chunk.FirstSequence >
  requestedFrom, so Task 11's pump can re-baseline after a ring-buffer
  overflow. Equality is NOT a gap.
- SampleAsync: ResponseHeadersRead + a hand-rolled
  multipart/x-mixed-replace frame reader (Content-length fast path,
  boundary-scan fallback), bounded by a heartbeat watchdog derived from
  HeartbeatMs so a silent Agent faults instead of wedging the pump - the
  S7 frozen-peer shape (arch-review R2-01). The long poll gets its own
  HttpClient with an infinite Timeout so the unary /probe + /current
  deadline stays bounded.

Task 0 package decision, option (c): both TrakHound PackageReferences and
their PackageVersions are dropped. MTConnectHttpClientStream takes a URL
and dials its own socket (no handler/stream seam, so it cannot serve a
socket-free unit suite), and it emits already-parsed documents through the
formatter lookup Task 6 proved is missing - framing-only reuse was never
actually on offer. The driver now depends on nothing but the BCL. Recorded
in the plan's Task 0 CORRECTION block.

Also folds in two Task 6 review carry-overs: the stale
IMTConnectAgentClient doc comment claiming a TrakHound-backed
implementation, and a probe test constructing a mixed-namespace vendor
extension (<x:Pump> with default-namespace <DataItems> children) that
backs the parser's ignore-the-namespace claim with a test.

187/187 green.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/MTConnectProbeParser.cs

@@ -11,8 +11,8 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.MTConnect;
 /// </summary>
 /// <remarks>
 ///     <para>
-///         <b>Why hand-rolled rather than TrakHound.</b> The referenced TrakHound packages
-///         (<c>MTConnect.NET-Common</c> + <c>-HTTP</c> 6.9.0.2) contain <b>no</b>
+///         <b>Why hand-rolled rather than TrakHound.</b> The TrakHound packages Task 0 pinned
+///         (<c>MTConnect.NET-Common</c> + <c>-HTTP</c> 6.9.0.2 — both dropped in Task 7) contain <b>no</b>
 ///         <c>IResponseDocumentFormatter</c> implementation — the XML formatter ships in the
 ///         separate, unreferenced <c>MTConnect.NET-XML</c> package — so
 ///         <c>ResponseDocumentFormatter.CreateDevicesResponseDocument("xml", stream)</c> answers