[Enhancement]: Tests — one guard is right by accident, and two failure paths have no test #2618

Problem

• The README's status-line count. ArchitectureTests checks the README's "announced on all 53 tabs that have one" by counting Text="{Binding StatusMessage}" plus <v:StatusFooter/> across Views/*.xaml (ArchitectureTests.cs:6002-6006). That counts StatusFooter.xaml's own TextBlock, which is not a tab, and misses Large Files' status line, which binds ScanStatus (Views/LargeFilesView.xaml:129). The two errors cancel out today, so the number is right for the wrong reason, and the next change to either breaks it in a confusing way.
• Restore All's NVIDIA card hand-off. Performance Mode's Restore All passes the card its question found to the restore (RestoreOriginalAsync(snapshot, card)), so the restore does what the question said. No test pins it, because every restore step before the graphics one writes this PC's registry and power settings.
• Environment Variables' unreadable backup. Services/EnvironmentVariableService.cs turns an IO, security or access failure while reading a safety copy into "unreadable". The tests reach the one case a test can cause, a legacy backup file held open by another handle. The other eight catches (the IO, security and access-denied ones around the User and Machine registry copies, and the legacy file's two access-denied ones) are reached by no test, because a test cannot make those reads fail on demand.

Proposed solution

• Count tabs, not markup: the guard maps each tab's view to whether it has an announced status line, so the number is the count of tabs.
• Give PerformanceService's restore steps a seam (the power plan, visual effects, Game Mode and Xbox writers), so a test can run Restore All against substitutes and assert the card it was handed is the one written.
• Read the safety copies through an injectable reader, so tests can make each read throw each exception type and assert the "unreadable" result.

Affected tab

Tests