[mrcsin]

Problem

The CSV file-load path (CsvService → CsvFileSerializer → CsvRowConverter) raised free-text Result.Fail(string) / new Error(string) for every deserialize failure, so a CSV import error rendered raw English regardless of culture. Worse, a bad cell showed just "Row 2": the untyped Row N wrapper stringified its inner, and ReasonLocalizer's fallback recursion dropped the column/value detail.

What changed

9 new typed errors + 2 reuses, so both the Localize and panel paths localize by type:

• AtRowError — a new composing row decorator (mirrors AtStepError). CsvRowConverter's column wrap reuses the slice-3 AtColumnError (byte-identical, already composes Localize(inner)), and unknown-action reuses 4b's ActionByIdNotFoundError. So the value chain composes AtRowError → AtColumnError → typed value error, rendering under ru as Строка 2: Столбец «gas»: Значение 5 больше максимума 4 для «amount» — Russian end to end.
• Leaf errors for action-column (not-found/empty), action-value-not-integer, CSV-body-empty, header-mismatch, recipe-file-not-found.
• CsvService IO/access failures → Rule-B exception-envelopes: a typed error carrying filePath, raised with .CausedBy(ex); the localized headline drops the raw ex.Message, which survives in the log via the existing _logger.LogWarning.

Behavior-preserving for English except three flagged changes: AtRowError gains : {inner} (the display fix), the ActionByIdNotFoundError reuse rewords unknown-action, and the two exception-envelopes drop : {ex.Message}. The CsvPropertyValidationTests.FlattenMessage gate asserts stay green (the full chain now lives in the composed .Message). Under ru the CSV import surface reads Russian.

Scope: CSV path only. Clipboard producer errors are slice 5c.

Verification

• dotnet build SemiStep.slnx — 0 warnings.
• dotnet test SemiStep/SemiStep.Tests/SemiStep.Tests.csproj — 1629 passed, 0 failed.
• The composition payoff: CsvImportLocalizationTests deserializes a bad-value CSV through the real CsvFileSerializer and asserts the full Russian chain under ru (both the Localize and the panel ReportFailure paths). ReasonLocalizerTests pins the ru render of all 9 new types + the en Localize == .Message invariant; the coverage test forces a case for each.
• Review chain: comprehensive (5 agents, all OUTCOME ACHIEVED) → smells → comment audit (Ship) → critical, all clean after two low fixes (ru render pins, a doc path).
• Manual test (operator): imported a CSV with an out-of-range cell / bad header under the Russian UI — the error shows in Russian with row/column context.

Per-task commits before collapse

1d5a1c8 docs: correct error namespace paths in error-reporting
1d484f3 test(l10n): pin russian render for remaining CSV import errors
7cdb343 test(l10n): cover CSV import error localization end-to-end
c3d9bb8 feat(l10n): type CSV serializer and service errors
f69bd74 feat(l10n): type CSV converter and row errors

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