Remove document protection before comparing

[SimonCropp]

Word.CompareDocuments blocks indefinitely on a document with enforced editing protection: Word waits on a "remove protection to compare the documents" prompt that DisplayAlerts cannot suppress. Because Launch only sets word.Visible after the compare, this leaves WINWORD.EXE alive with no visible window.

Word.Open now strips protection from each in-memory copy before the compare. The documents are opened ReadOnly and never saved, so the files on disk are untouched. The empty-password Unprotect overload throws rather than showing its own blocking prompt, so a password- protected input surfaces as a logged failure instead of another hang.

Also fixes two pre-existing test-suite issues that this surfaced:

• SourceDocumentsVisibleTests leaked a WINWORD process (unreleased child-document COM references prevented Quit from reaping it); it now tracks and kills the process it spawns.
• The process-counting ProcessCleanupTests ran in parallel with the Word-spawning tests and observed their processes; the Word-touching classes now share [NotInParallel("MicrosoftWord")].