test: add Clear concurrency tests (Clear during Store, Clear during Range)

[millerjp]

Summary

TestClear covers Clear only single-threaded. A consumer calling Clear concurrently with Store/Load/Range (cache-flush-on-reload patterns) has no safety evidence.

Scope

Analogous to existing TestRangeDuringWrites / TestDeleteDuringRange:

• TestClearDuringWrites — 16 goroutines storing while one goroutine periodically Clears; assert no panic, -race clean, goleak clean.
• TestClearDuringRange — one ranger running while Clear fires; assert no panic, observed keys bounded, duplicate visits impossible.

Acceptance criteria

1. Both tests stable over go test -race -count=10 ./....
2. Coverage unchanged (100%).

Source: test-analyst agent.