Add a frequency tie-break and per-match frequencies to semsim search - #1484

Closes #1478. Part of #1474. Based on #1483.

Summary

tie_break (none default, or frequency) and default_frequency (0.5) on search.

• With frequency, entities with equal scores are ordered by how frequent the matched phenotypes are in each entity. For each query term, the harness takes the frequency of the entity annotation that best matches it, and sums those in log space.
• Where frequency comes from:
  • OMIM has_quotient, such as 9 of 12 patients, when present.
  • Otherwise the midpoint of the HPO frequency category, such as "Occasional" giving 0.17.
  • Otherwise default_frequency.
  • Values are floored at 0.01, because some OMIM edges record 0 patients.
• The tie-break runs inside the ranking query. So limit keeps the best of a tie group, not its first IDs.
• Each best match in a ducksim search now carries frequency, the matched annotation's recorded frequency when it has one.

Design notes

• Frequencies are computed per query. They come from a _freq view over edges, evaluated over only the entities that query touches, so nothing is computed at startup. The other option in semsim API: frequency tie-break for phenotype search #1478 was a normalised column built into the KG. That's still available if Solr or the UI need the numbers elsewhere.
• Coverage. About 80% of disease-phenotype edges carry a frequency. Of human gene edges, 101,236 of 158,412 do. Model organism gene edges carry none, so there the tie-break has no effect.
• A KG whose edges lack the frequency columns gets an empty view, and the tie-break falls back to ID order.

Testing

• The backend suite passes: 793 passed, 16 skipped. The new fixture KG has four diseases tied on one phenotype, with frequencies of 0.9, "Occasional", none recorded, and 0 of 5. The tests cover ordering, default_frequency, the tie-break applying before the limit, per-match frequencies, KGs without frequency columns, and the API.
• Checked against monarch-kg 2026-10-07, with full search and query-to-disease scoring. SLC35A2-CDG's position within the tie group:

https://claude.ai/code/session_01RmR3yV2ogn7Rx7bJnXXouU