Reduce vacuous and brittle recall-related test patterns

[Lyellr88]

Summary

Harden recall-related tests so they fail loudly on empty or unstable states instead of passing vacuously.

Discussion: #49

Why this matters

Recent work found a few recall tests that could pass on empty results or become brittle under module reloads. The suite is much better now, but this is a good place for a contributor to keep tightening things.

Good first scope

• Audit recall and hybrid-search tests for all([]) and if results: patterns that hide failures
• Replace vacuous checks with explicit non-empty assertions where appropriate
• Reduce brittle monkeypatch patterns when module reloads or isolated imports are involved
• Keep behavior unchanged. This is test hardening only.

Candidate files

• tests/test_hybrid_search.py
• tests/test_temporal_weighting.py
• tests/test_memory_database.py
• tests/test_3layer_retrieval.py
• related recall tests if needed

Acceptance ideas

• Tests fail loudly when setup is wrong or results are empty
• Focused recall tests still run fast
• No broad test harness rewrite required

[Lyellr88]

Being handled by adity982 in PR #124