Adds DiffDispatcherGuardTests, the one blockade-regression test in the
operation-feedback effort. Uses a queueing SynchronizationContext (Option B —
this project has no bootstrapped Avalonia dispatcher anywhere) to prove
DiffViewerViewModel's UnifiedDiffParser.Parse offload ([B1]) truly defers past
the calling thread rather than racing on timing. [B2]'s DiffAlignment.Build
offload in DiffTextView is deliberately not covered — three escalating
attempts to construct that control safely in-process each broke under the
full test suite (dispatcher thread-affinity crashes, then process-wide
Application state polluting an unrelated test), and Avalonia.Headless.XUnit
collides with this project's xUnit v2 stack. Verified the test fails against
the pre-[B1] code and passes with it; full suite green aside from a
pre-existing, unrelated flaky test.
Mika Kuns
c795827162