gRPC M0: probe (R0.4, live-verified) + system-param (R0.3) + shared handshake

Roadmap docs/plans/hcal-roadmap.md, milestone M0 (gRPC parity for the DONE
surface). Now unblocked for live verification by a reachable 2023 R2 server.

- R0.4 Probe over gRPC: new HistorianGrpcProbe calls History/Retrieval/Status
  GetInterfaceVersion (unauthenticated). ProbeAsync routes over gRPC when
  Transport==RemoteGrpc. LIVE-VERIFIED against a real 2023 R2 server — needs no
  credentials (runs before the auth loop), so it works despite the auth blocker.

- R0.3 System parameter over gRPC: new HistorianGrpcStatusClient calls
  StatusService.GetSystemParameter over the authenticated session; routed in the
  dialect. Built + unit-tested (request/response field mapping pinned).
  Live-verification pending an auth fix (see below).

- Extracted the proven auth handshake from HistorianGrpcReadOrchestrator into
  shared Grpc/HistorianGrpcHandshake (reused by read + status + future
  browse/metadata). Repointed the IL structural guardrail test to it.

- Diagnostics: round-failure now decodes the native server error + hex/ASCII
  preview (HistorianNativeHandshake.DescribeError). This surfaced the live auth
  blocker as SEC_E_LOGON_DENIED (0x8009030C) at NTLM round 1 — framing is correct,
  the credential did not validate. Probable cause: stale file password or NAM-domain
  NTLM restriction (Kerberos/RDP works, NTLM denied; no SPN path over the tunnel).

216 unit tests pass; live gRPC probe passes. Sanitization scan clean.

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

src/AVEVA.Historian.Client/Protocol/Historian2020ProtocolDialect.cs

@@ -64,7 +64,9 @@ internal sealed class Historian2020ProtocolDialect
     {
         cancellationToken.ThrowIfCancellationRequested();
         ArgumentException.ThrowIfNullOrWhiteSpace(name);
-        return Wcf.HistorianWcfStatusClient.GetSystemParameterAsync(_options, name, cancellationToken);
+        return UseGrpc
+            ? HistorianGrpcStatusClient.GetSystemParameterAsync(_options, name, cancellationToken)
+            : Wcf.HistorianWcfStatusClient.GetSystemParameterAsync(_options, name, cancellationToken);
     }
 
     private static async IAsyncEnumerable<T> Missing<T>(