[mrcsin]

Problem

The panel had localizing seams for a Result's errors (ReportFailure(IResultBase)) and warnings (ReportWarnings, slice 5a), plus the snapshot-validity channel (RefreshReasons) — but none for a single transient IError. The PLC fault channel carries a typed IError end to end (PlcSyncCoordinator.EmitFault → Faults → RecipeCoordinator.OnPlcFault), yet OnPlcFault flattened it to .Message into the raw ReportError(string) sink — so the fault rendered raw English regardless of culture, and the one fault it carries was an untyped new Error("PLC connection lost") anyway.

What changed

The error-side twin of slice 5a's warning seam, proven on the connection-lost fault:

• ReportFailure(this MessagePanelViewModel, IError error) — a localizing overload mirroring ReportFailure(IResultBase): panel.ReportError(ReasonLocalizer.Localize(error)). IError is not IResultBase, so the overload is unambiguous (all existing callers pass Results). The panel now has all four seams: ReportFailure(IResultBase), ReportFailure(IError), ReportWarnings, RefreshReasons.
• ConnectionLostError (typed) replaces PlcSyncCoordinator's new Error("PLC connection lost").
• OnPlcFault routes through the seam, keeping the English _logger.LogWarning("PLC fault: {Message}", ...) (log-English invariant).

Behavior-preserving for English: ConnectionLostError's base message equals today's string and the resx en value equals it byte-for-byte. Under ru the PLC fault now reads Russian. The seam localizes any IError — an untyped inner still falls through to .Message, so nothing else regresses.

Scope: this is the small mechanism cut of the PLC slice. The bulk error typing + un-laundering PlcSyncExecutor (6b) and the #120 cancellation plumbing (6c) follow.

Verification

• dotnet build SemiStep.slnx — 0 warnings.
• dotnet test SemiStep/SemiStep.Tests/SemiStep.Tests.csproj — 1643 passed, 0 failed.
• ResultReportingExtensionsTests drives ReportFailure(new ConnectionLostError()) under ru (Russian in the operation slot, Error severity) + the untyped-fallthrough (.Message). PlcSyncCoordinatorTests pins BeOfType<ConnectionLostError>() at the emit source; PlcFault_RoutesToMessagePanelOperationChannel (en-scoped) keeps the routing/severity contract. ReasonLocalizerTests pins the ru render + en Localize == .Message; the coverage test forces the case.
• Review chain: comprehensive (5 agents, all OUTCOME ACHIEVED) → smells → comment audit (Ship) → critical, all clean after two low fixes (a doc qualifier, a brace-style alignment).
• Manual test (operator): dropped the PLC connection under the Russian UI — the fault shows in Russian in the operation channel.

Per-task commits before collapse

c99814c refactor(core): align ConnectionLostError to leaf-error brace style
9d2af24 docs: qualify ReportFailure overload in seam note
0b1bd01 test(l10n): cover transient-error seam and PLC connection-lost render
d3a86db feat(l10n): localize PLC connection-lost fault via transient seam

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