feat(l10n): localize grid-style editor errors by type

[mrcsin]

Problem

The in-app grid-style editor (GridStyleEditorViewModel) was the last unlocalized operator surface in the error-localization roadmap. Its ErrorMessage was built by joining raw .Message strings, bypassing ReasonLocalizer entirely — ReasonLocalizer wasn't referenced in StyleEditor/ at all. So a load / validate / save failure rendered English regardless of UI culture. The editor opens after startup (culture established), so there's no reason to leave any of it English — leaving loader errors English while validator errors were Russian would show one panel in mixed languages depending on whether the file was missing or a color was malformed.

What changed

Type all 8 grid-style error producers and route the three ErrorMessage join sites through ReasonLocalizer.Localize:

Producer	Typed errors
GridStyleValidator	config-missing, section-missing (unifies colors + generic), orientation-invalid, key-missing, invalid-hex-color
GridStyleLoader	config-not-found; load-failed (Rule-B envelope)
GridStyleWriter	save-failed (Rule-B envelope)

Each gets a ReasonLocalizer arm, en/ru resx + Designer, and a coverage sample. The join sites localize by type and fall through to .Message for anything not yet typed. English base messages stay byte-identical except the two Rule-B headlines, which drop the : {ex.Message} tail.

Two design points worth calling out:

• The orientation error carries its two expected values as properties — GridOrientationValues is internal to Core and invisible to the UI localizer, so the arm can't reference it.
• The dropped exception detail rides CausedBy(ex) and is logged at both consumers of the shared loader: the editor view model (LogWarning) and the startup path in Program.cs (Log.Error). Without the startup sink, a malformed grid_style.yaml at launch would lose its parse detail entirely.
• ReportSaveException drops its raw tail to the already-localized Resources.SaveFailed — the exceptions reaching it are pre-write pipeline throws, not file-write failures, so naming a file would mislead.

Scope: pure localization. Issue #118 (the editor's structural refactor — declarative 60-field map, async Save, HexColor.Parse removal) is independent and untouched; only the three join sites move. After this, the operator-facing error surfaces are fully typed — the startup config load (before culture exists) and the two PLC malformed-wire diagnostic edges remain English by design.

Verification

• dotnet build SemiStep.slnx — 0 warnings; dotnet test — 1685 passed, 0 failed; dotnet format clean.
• ReasonLocalizerTests pins each of the 8 types' ru render + the en Localize == .Message pins; CoreErrorLocalizationCoverageTests auto-enrolls the 8 public errors; ResourceSyncTests enforces en/ru/Designer key + placeholder parity.
• GridStyleEditorViewModelTests.LoadAsync_MalformedHexColor overlays a corrupted color on the shipped config and asserts ErrorMessage is the exact Russian hex sentence (a regression to raw .Message fails it). Two log-sink tests assert the CausedBy exception reaches the log — removing the sink call fails them.
• GridStyleEditorFacadeTests assert GridStyleLoadFailedError/GridStyleSaveFailedError retain their exception on the nested .Reasons.
• Review chain: comprehensive (5 agents, all OUTCOME ACHIEVED) → smells → comment audit → critical, all clean after fixes. The reviews caught an untested log sink (added tests), a doc overreach ("sole English-by-design surface" — qualified against the PLC edges), and a Path→FilePath rename.
• Manual test (operator): passed.

Per-task commits before collapse

99524a9 docs(plan): record exec branch and verify steps for slice 7
0ab4468 docs(test): drop misleading Rule-B comment in localizer test
4143204 refactor(l10n): rename GridStyleConfigNotFoundError.Path to FilePath
70409ad test(l10n): cover grid-style exception log sink; fix doc overreach
44a75bf test(l10n): cover grid-style error localization end-to-end
79c7adf feat(l10n): type grid-style file-IO errors
713a808 feat(l10n): type grid-style validation errors

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