* test(notifications): reproduce lost incident identity after partial status

* test(notifications): cover partial incident identity through planner priming

* fix(notifications): retain incident identity across partial status replies

Keep the last known nonempty incident id while an active status temporarily
omits it. Use the same memo transition during silent priming so a settings
reset cannot hide a later replacement incident either.

Preserve current severity, cold-start silence, and authoritative recovery.
Add the Unreleased note in this production commit.

The test-only parent reproduces all three failures in native Qt via make
check: 998 passed, 3 failed, 0 skipped (CI run 34594803546).