fix(search): bound query result caches

[damienriehl]

Summary

Long-running consumers no longer retain results for every unique label, definition, and prefix query. Search behavior stays unchanged, while process-lifetime query caches retain at most 128 entries each.

A folio-resolve synthetic workload exposed the impact. After one passage, _basic_search held 1,707 entries and the case-insensitive prefix cache held 766 entries; process RSS reached 1.29 GiB. The same unbounded growth helped drive a 225-item run to a 30.48 GB peak.

Validation

• Full suite: 80 passed with 86% coverage.
• Ruff: clean.
• The regression drives 129 unique queries through all three cache families and verifies each stops at 128 entries.