Nonauthoritative observations cleared the last notified quota severity before
skipping quota processing. An unchanged measured quota consequently repeated
its warning after a temporary estimated or unknown-confidence reading.
Carry the severity until measured usage recovers or escalates, preserving
reset disarming and the existing bounded scope identity.

Add four adversarial regression cases, verified red before and green after.
Documentation impact: update usage and development contracts and Unreleased;
TODO has no affected parity entry or CLI blocker. No strings or defaults change.

Verification: make check JOBS=8 passed (2556 QtTest passes, 110 optional KDE
skips; 374 Python tests, one optional KDE skip; partial local Plasma QML type
metadata checks). Focused planner/confidence tests, real estimated-usage smoke,
make package, and git diff --check passed.