Function keyword_search 

pub async fn keyword_search(
    conn: &Connection,
    query: &str,
    top_k: usize,
    as_of_valid: Option<&str>,
    half_life: Option<Duration>,
) -> Result<Vec<(String, f64)>>

Keyword search over concept text, best match first (§5.9).

Ranked by bm25, which FTS5 returns as a negative number whose magnitude grows with relevance, so ascending order is best-first. Retired concepts are excluded: a soft-deleted concept is not a search result, and the index cannot filter on retired itself because external-content FTS5 indexes only the columns it was declared over.

The visibility predicate is the vector arm’s, spliced rather than repeated (0.13.19, W9.4, D-192). This function carried its own AND c.retired = 0 from the day it was written, and W9.3 wrote the shared constant without folding this copy into it. Two literals that must agree is D-030’s failure class, and W9.4 is the release that would have made them disagree: adding the valid-time bound to one and not the other is F-31 again with a different column.

as_of_valid bounds each hit against its own valid interval. Absent, the statement is what 0.13.18 issued. FTS5 is not consulted about it either way: the MATCH selects on text and the bound is applied to the joined concepts row, which is the only place either fact lives.

The join names c.rowid_pk rather than c.rowid (v8, D-119). They are the same value — an INTEGER PRIMARY KEY is the rowid — but concepts_fts declares content_rowid='rowid_pk', and the join should say which key it is joining on rather than rely on the alias holding.