Add a focused local-library test for the case where a game starts with only a
committed `local/` install and later gains a root `version.ini` sentinel. The
single-game rescan now has coverage showing it promotes the summary from
LocalOnly to Ready while preserving the installed flag and reading the local
version.
This pins the cached-index transition called out in FOLLOW_UP_2.md without
touching scanner dispatch or broader monitor behavior.
Test Plan:
- git diff --check
- just fmt
- just clippy
- just test
Follow-up-Plan: FOLLOW_UP_2.md
ddidderr
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