fix: preserve Unified Search empty state

[AJV20]

Summary

• keep the initial Unified Search guidance visible while a query is only being typed
• show “No Results” only after a successful completed search
• invalidate stale results and in-flight work when the query, sources, backup, or cancellation state changes
• add a compiled behavioral regression for typed, completed-empty, changed-query, and stale-completion transitions

Verification

• python3 Scripts/regression/run.py — 155 checks passed
• swift build
• swift build -c release
• bash Scripts/build.sh
• live macOS UI verification against an actual backup
• independent exact-diff P0/P1 review — PASS