Add dispatch-level tests for the local game monitor paths called out in
FOLLOW_UP_2.md. The new coverage verifies watcher events are dropped while a
game has an active operation, burst events for one game collapse through the
pending set to at most one extra rescan, fallback scans pick up sideloaded
catalog games, and non-catalog roots stay invisible to the library state.
The non-catalog test exposed that an empty local library initialized with
digest zero, while the computed digest for an empty map is nonzero. That made
the first empty scan produce a meaningless empty LibraryDelta. Initialize the
empty state with the computed empty digest so a non-catalog-only scan leaves no
delta behind.
Test Plan:
- git diff --check
- just fmt
- just clippy
- just test
Follow-up-Plan: FOLLOW_UP_2.md
ddidderr
2a94445391
