refactor(config): move grid-style Save off the UI thread

[mrcsin]

Problem

The grid-style editor's Load is async, but Save was synchronous — SaveCommand = ReactiveCommand.Create(Save, …) did file I/O (File.WriteAllText + File.Move) on the UI thread, blocking the dispatcher on every save. Slice 2 of the #118 grid-style debloat.

What changed

Make the write path async end to end, matching Load:

• IGridStyleEditorFacade.Save → Task<Result>; GridStyleEditorFacade.Save awaits GridStyleWriter.SaveAsync.
• GridStyleWriter — the temp-file write and header read use WriteAllTextAsync / ReadAllLinesAsync; File.Move stays synchronous inside the atomic-write try/catch (there is no async move, and it's a fast metadata rename). The slice-7 Rule-B GridStyleSaveFailedError.CausedBy(ex) envelope is preserved; the private helpers gained the Async suffix (WriteAtomicAsync/ReadLeadingCommentBlockAsync).
• GridStyleEditorViewModel — Save → SaveAsync, wired via ReactiveCommand.CreateFromTask. SaveCommand stays ReactiveCommand<Unit, bool>, so the window's save→restart-prompt→close flow and the ThrownExceptions → ReportSaveException routing are unchanged (GridStyleEditorWindow.axaml.cs is untouched).

Behavior-preserving: the bytes written are byte-identical — only the write's threading changes. Validate stays synchronous (it runs per-keystroke in RecomputeCanSave), no CancellationToken is threaded (Save is a button-click), and slice 1's round-trip guards still hold. Nothing from slices 3–5 (record nesting, StyleColor, the VM split) leaks in.

Verification

• dotnet build SemiStep.slnx — 0 warnings; dotnet test — 1687 passed, 0 failed; dotnet format clean.
• Slice-1 guard SaveThenLoad_DistinctFixture… (now awaiting the async Save) is green — proving identical bytes. The atomic-write / File.Move-failure test and the save-fault tests (ThrowingFacade's synchronous throw → ReportSaveException; CausedByFailingFacade → LogCausedByExceptions) still fire after the conversion.
• Review chain: comprehensive (5 agents, all OUTCOME ACHIEVED) → smells → comment audit → critical, all clean after one fix (the async-helper suffix rename). A second finding — the facade's Save lacking the Async suffix — was correctly kept: the facade's Load is also suffix-less-async, so Save matching it is the consistent choice.
• Manual test (operator): confirmed.

Slice 2 of #118 (stays open). Next: slice 3 nests GridStyleOptions to mirror the DTO groups — the big line-count win.

Per-task commits before collapse

0b6b170 docs(plan): record exec branch and verify steps for async save
33adcad refactor(config): suffix grid-style writer async helpers
61d3aa2 docs(config): note grid-style Save is now async
06c4f72 refactor(config): make grid-style Save async off the UI thread

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