fix: report an unreadable disk as unread rather than degrading

[laurentiu021]

Closes #2099.

What was wrong

HealthScoreService marked the disk component unavailable only when the list was empty:

if (disks is null || disks.Count == 0) unavailable.Add(DiskComponent);

A drive with no SMART data still produces a DiskHealthReport — HealthPercent is documented to return null in that case — so the list is non-empty, the component is not marked unavailable, ComputeDiskScore returns the deliberate unknown 80, and ClassifySmartHealth(80, unavailable: false) lands in the >= 60 branch: "Disk health degrading — check System Health", yellow.

The codebase already ruled that outcome out in two places:

• DashboardViewModel.ClassifySmartHealth's remarks: "Scoring it as unknown instead would put it in the 'degrading' branch, which is a different wrong answer: nothing is degrading, nothing was measured."
• UnknownComponentScore_StaysBelowEveryGreenBranch asserts >= 60 with the comment "and must not read as degrading either — nothing was measured" — an intent this path did not deliver.

Not rare: consumer SATA and NVMe drives frequently expose nothing through MSFT_StorageReliabilityCounter, and a VM exposes nothing at all.

The fix

disks.All(d => d.HealthPercent is null) — drives present but none readable is the same absence of evidence as no drives at all.

The decision moved into a pure internal static UnavailableComponents(disks, snapshot), for the same reason ComputeDiskScore is one: it is worth asserting, and asserting it through ComputeAsync would mean querying WMI.

All, not Any, deliberately. With one drive readable at 20% and one unreadable, Any would mark the component unavailable and replace "Disk health critical" with "could not be read" — hiding a drive Windows has already flagged as failing. A mixed read keeps the worst measured verdict.

Verification

Two mutations, restored byte-for-byte and re-hashed:

Mutation	Result
back to disks.Count == 0	RED on the new all-unreadable test — Collection: ["Memory", "Uptime"] / Not found: "Disk"
All → Any	RED on the mixed-read test (Collection: ["Disk", …] / Found: "Disk") and on the empty-list test

That second row corrected my own prediction and is worth recording: [].All(…) is vacuously true while [].Any(…) is vacuously false, so All covers the no-drives case for free and Any would have silently broken it. Both reasons point the same way.

Baseline and post-restore: 4 green, 0 red. Regression sweep: 230 named tests across HealthScoreServiceTests, DashboardViewModelTests, DiskHealthReport*, SystemHealthViewModelTests and ArchitectureTests — 350 cases green, plus the harness-only author-header case for the throwaway runner. Builds 0 errors / 0 warnings, dotnet format --verify-no-changes clean on both projects, version consistency csproj 1.76.11 = CHANGELOG 1.76.11 = SECURITY 1.76.x.

Docs

No README change: the tab's behaviour is described in terms of what it reports, and the README does not name the degrading wording. CHANGELOG entry included.