fix: confirm notification delivery before deduplication

[abrichr]

Summary

• confirm the desktop notification backend accepts a notification before reporting delivery
• keep a failed lifecycle notification eligible for retry
• preserve lifecycle de-duplication across unmessaged transition states

Verification

• 235 passed, 2 skipped
• focused notification/application tests: 40 passed
• Ruff checks pass on changed files
• wheel and sdist build
• source-boundary and release-consistency gates pass
• independent review: no remaining high- or medium-severity issue