docs: name each screenshot after its tab instead of a position number

[laurentiu021]

Problem

The 43 files in docs/screenshots/ were prefixed with the tab's position on the left rail, and that prefix re-broke on every insertion: adding one tab shifts every tab below it and touches no filename. 23 of the 43 were numbered for a different tab than the one they showed, one of them by eight places — and nothing could notice, because a wrong number looks exactly like a right one. TheScreenshotInventory_MatchesWhatIsOnDisk says so in its own remarks: it deliberately did not check which screenshot covers which tab, because with a number in the name there was no rule to check against.

Three filenames also named a tab that no longer goes by that name, the newest being from the previous release.

What changed

Naming. Each file is named for its tab alone: the sidebar label lowercased, with each run of non-alphanumeric characters collapsed to one hyphen. 42-debloater.png → preinstalled-apps.png, 21-app-alerts.png → new-app-alerts.png, 22-file-lock.png → file-lock-detector.png, 16-cpu-affinity.png → cpu-core-affinity.png, 14-standby-cleaner.png → standby-list-cleaner.png, and 38 more. Adding a tab now touches nothing that already exists. All 43 are pure renames — git reports 100% similarity on every one, no image was re-encoded.

A new guard, which is what the convention buys. EveryScreenshotSlug_NamesARealTab:

• every filename on disk must reduce from a real sidebar label;
• every gallery image's alt text must name the same tab its filename does;
• two tabs reducing to one slug is reported as its own failure, because that would leave one of them permanently unable to have a shot.

The alt text is the half that matters most. It is what someone using a screen reader gets instead of the picture, so an alt text left on an old name has nothing else to correct it, while a sighted reader can at least read the header inside the image. All 43 are correct today, which is why the assertion was free to add.

The cost, stated on purpose. Renaming a tab leaves its screenshot orphaned until the file is renamed too. That is the point rather than a side effect: a renamed tab means the header rendered inside the image says the old name as well, so the file needs recapturing and not just moving. The guard names the file that has to change, and docs/screenshots/README.md now says so where a contributor will read it.

Two stale doc comments. The inventory guard's remarks explained why the slug check could not be written; the tab-count regex cited 52-system-logs.png as the shape its "tabs" requirement exists to exclude. Neither shape exists any more.

Verification

Red ritual on the new guard — all four cases red for the right reason, green after restore, every file restored from saved bytes rather than from git:

Mutation	Result
a screenshot renamed to a name no tab has	RED, names the orphan
an alt text captioned for a different tab	RED, names the file and the caption
a tab renamed in MainWindowViewModel, no file touched	RED, names battery-health.png and says to recapture
the gallery markup stops parsing	RED on the vacuity floor, not a silent empty loop

• Unit suite: 5775 passed, 0 failed (5774 before, +1 new guard).
• dotnet build -c Release on app + tests: 0 errors, 0 warnings.
• dotnet format --verify-no-changes: clean.
• git grep finds no surviving numbered screenshot reference anywhere in the tree.
• Line endings unchanged on all three text files (i/lf w/crlf, as tracked).

Scope

Refs #1664 — this settles the naming half. The 16 missing captures stay open; they need a machine that runs the application.