[ProGraMajster]

Summary

• Probe Classic WinRT notification availability before selecting it; cache native provider failure and advance once through policy-allowed backends.
• Reclaim stale Shell notification identities with bounded dispatcher-owned leases, replacement and late-callback guards.
• Add deterministic regression tests and document capability/fallback/lifetime semantics.

Refs #269, #270. Prepared for the ModernTubeDownloader storage/notification hotfix; no framework version bump or publication.

Validation

• Full solution Debug and Release builds: zero warnings/errors (sequential).
• Notification subsets: 63 Windows + 37 portable contracts in each configuration; scroll/layout subset: 34 in each.
• Fresh pre-push complete Windows backend Release suite: 410 passed, zero failures/skips.
• Isolated native consumer fallback smoke with optional providers absent selected Shell and accepted one submission; no banner/click or physical Windows 11 23H2 certification is claimed.

Publication boundary

Source push/PR only. No tag, GitHub Release, NuGet publication or protected-branch bypass.