feat(ui): route command exceptions through report+log pipe

[mrcsin]

Problem

Command-exception handling was eight copies of the same
ThrownExceptions → ObserveOn → panel.ReportError block, and none of them
logged. A user saw the panel message, but the exception type and stack never
reached the log — a field-reported error left nothing to diagnose with. Two
paths were worse than silent: three MainWindowViewModel subscriptions had no
guard (a throw in the callback escaped to the dispatcher, fatal for the
per-second sync-time tick), and GridStyleEditorViewModel.SaveCommand had no
ThrownExceptions handler at all — an unexpected save fault crashed the process.

What changed

• Extract one ReportThrownExceptions(command, panel, logger, context) extension
  that reports to the panel and logs the exception with its stack via the
  caller's ILogger<T>, so the log line carries the originating class through
  {SourceContext}. All eight sites route through it; the redundant ObserveOn
  is dropped (the panel self-marshals via PostOnUiThread).
• Guard the three bare MainWindowViewModel subscriptions so a throw in the
  callback body is reported instead of escaping.
• Wire GridStyleEditorViewModel.SaveCommand faults to the editor's own
  ErrorMessage (a modal fault must not land behind the modal) plus the log.
• No IMessageSink interface: MessagePanelViewModel has one implementation, is
  not mocked, and crosses no layer boundary, so consumers keep the concrete panel.
  The one interface introduced — IGridStyleEditorFacade — is warranted (it is now
  mocked in a test and crosses the Core→UI boundary).

No happy-path behaviour change: on success nothing differs; on failure the same
panel message appears and a stack-carrying log line is added.

This is PR-F1 of the error/logging pipe roadmap. Follow-ons: PR-F2 (global
exception backstop), then #113 rebased on both.

Verification

• dotnet build SemiStep.slnx — 0 warnings (warnings are errors).
• dotnet test SemiStep/SemiStep.Tests/SemiStep.Tests.csproj — 1474 passed, 0 failed (+4 new tests).
• dotnet format clean; Rider analyzer clean on all changed files.
• New regression tests reproduce each closed fault: a throwing command reports and logs the exception; SaveCommand.Execute() through a throwing facade surfaces on ErrorMessage + log without crashing; a throw inside a guarded onNext body is contained.
• Ran through the full multi-agent review chain (comprehensive, smells, comment audit, critical); two MAJOR findings caught and fixed (subscription guards only caught source errors, not callback-body throws; SaveCommand wiring was tested only at the seam).

Per-task commits before collapse

1701f5d feat(ui): add report+log command-exception extension
b71f7eb feat(ui): route ClipboardViewModel command errors through report+log extension
b28fe4f feat(ui): route RecipeFileViewModel command errors through report+log extension
3b4da06 feat(ui): guard MainWindowViewModel command and subscription errors
f1e683b feat(ui): surface and log GridStyleEditor save faults on its own error surface
6fa2e20 docs(architecture): document report+log command-exception pipe
92d0580 fix(ui): address review findings — guard onNext throws, test wiring, log/doc fixes
94a046e refactor(ui): use shared test logger and IGridStyleEditorFacade seam
67d1937 refactor(ui): drop redundant doc comment on ExceptionReporter
2461a5b docs(plans): record exec branch + verification steps for PR-F1