FOCAS Tier-C PR C — IPC path end-to-end: Proxy IpcFocasClient + Host FwlibFrameHandler + IFocasBackend abstraction. Third of 5 PRs for #220. Ships the wire path from IFocasClient calls in the .NET 10 driver, over a named-pipe (or in-memory stream) to the .NET 4.8 Host's FwlibFrameHandler, dispatched to an IFocasBackend. Keeps the existing IFocasClient DI seam intact so existing unit tests are unaffected (172/172 still pass). Proxy side adds Ipc/FocasIpcClient (owns one pipe stream + call gate so concurrent callers don't interleave frames, supports both real NamedPipeClientStream and arbitrary Stream for in-memory test loopback) and Ipc/IpcFocasClient (implements IFocasClient by forwarding every call as an IPC frame — Connect sends OpenSessionRequest and caches the SessionId; Read sends ReadRequest and decodes the typed value via FocasDataTypeCode; Write sends WriteRequest for non-bit data or PmcBitWriteRequest when it's a PMC bit so the RMW critical section stays on the Host; Probe sends ProbeRequest; Dispose best-effort sends CloseSessionRequest); plus FocasIpcException surfacing Host-side ErrorResponse frames as typed exceptions. Host side adds Backend/IFocasBackend (the Host's view of one FOCAS session — Open/Close/Read/Write/PmcBitWrite/Probe) with two implementations: FakeFocasBackend (in-memory, per-address value store, honors bit-write RMW semantics against the containing byte — used by tests and as an OTOPCUA_FOCAS_BACKEND=fake operational stub) and UnconfiguredFocasBackend (structured failure pointing at docs/v2/focas-deployment.md — the safe default when OTOPCUA_FOCAS_BACKEND is unset or hardware isn't configured). Ipc/FwlibFrameHandler replaces StubFrameHandler: deserializes each request DTO, delegates to the IFocasBackend, re-serializes into the matching response kind. Catches backend exceptions and surfaces them as ErrorResponse{backend-exception} rather than tearing down the pipe. Program.cs now picks the backend from OTOPCUA_FOCAS_BACKEND env var (fake/unconfigured/fwlib32; fwlib32 still maps to Unconfigured because the real Fwlib32 P/Invoke integration is a hardware-dependent follow-up — #220 captures it). Tests: 7 new IPC round-trip tests on the Proxy side (IpcFocasClient vs. an IpcLoopback fake server: connect happy path, connect rejection, read decode, write round-trip, PMC bit write routes to first-class RMW frame, probe, ErrorResponse surfaces as typed exception) + 6 new Host-side tests on FwlibFrameHandler (OpenSession allocates id, read-without-session fails, full open/write/read round-trip preserves value, PmcBitWrite sets the specified bit, Probe reports healthy with open session, UnconfiguredBackend returns pointed-at-docs error with ErrorCode=NoFwlibBackend). Existing 165 FOCAS unit tests + 24 Shared tests + 3 Host handshake tests all unchanged. Total post-PR: 172+24+9 = 205 FOCAS-family tests green. What's NOT in this PR: the actual Fwlib32.dll P/Invoke integration inside the Host (FwlibHostedBackend) lands as a hardware-dependent follow-up since no CNC is available for validation; supervisor + respawn + crash-loop breaker comes in PR D; MMF + NSSM install scripts in PR E.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Host/Backend/UnconfiguredFocasBackend.cs

@@ -0,0 +1,37 @@
+using System.Threading;
+using System.Threading.Tasks;
+using ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Shared.Contracts;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Host.Backend;
+
+/// <summary>
+///     Safe default when the deployment hasn't configured a real Fwlib32 backend.
+///     Returns structured failure responses instead of throwing so the Proxy can map the
+///     error to <c>BadDeviceFailure</c> and surface a clear operator message pointing at
+///     <c>docs/v2/focas-deployment.md</c>. Used when <c>OTOPCUA_FOCAS_BACKEND</c> is unset
+///     or set to <c>unconfigured</c>.
+/// </summary>
+public sealed class UnconfiguredFocasBackend : IFocasBackend
+{
+    private const uint BadDeviceFailure = 0x80550000u;
+    private const string Reason =
+        "FOCAS Host is running without a real Fwlib32 backend. Set OTOPCUA_FOCAS_BACKEND=fwlib32 " +
+        "and ensure Fwlib32.dll is on PATH — see docs/v2/focas-deployment.md.";
+
+    public Task<OpenSessionResponse> OpenSessionAsync(OpenSessionRequest request, CancellationToken ct) =>
+        Task.FromResult(new OpenSessionResponse { Success = false, Error = Reason, ErrorCode = "NoFwlibBackend" });
+
+    public Task CloseSessionAsync(CloseSessionRequest request, CancellationToken ct) => Task.CompletedTask;
+
+    public Task<ReadResponse> ReadAsync(ReadRequest request, CancellationToken ct) =>
+        Task.FromResult(new ReadResponse { Success = false, StatusCode = BadDeviceFailure, Error = Reason });
+
+    public Task<WriteResponse> WriteAsync(WriteRequest request, CancellationToken ct) =>
+        Task.FromResult(new WriteResponse { Success = false, StatusCode = BadDeviceFailure, Error = Reason });
+
+    public Task<PmcBitWriteResponse> PmcBitWriteAsync(PmcBitWriteRequest request, CancellationToken ct) =>
+        Task.FromResult(new PmcBitWriteResponse { Success = false, StatusCode = BadDeviceFailure, Error = Reason });
+
+    public Task<ProbeResponse> ProbeAsync(ProbeRequest request, CancellationToken ct) =>
+        Task.FromResult(new ProbeResponse { Healthy = false, Error = Reason, ObservedAtUtcUnixMs = System.DateTimeOffset.UtcNow.ToUnixTimeMilliseconds() });
+}