Function fts_search_symbols_explained 

pub fn fts_search_symbols_explained(
    conn: &Connection,
    query: &str,
    kind_filter: Option<&str>,
    path_filter: Option<&str>,
    include_tests: bool,
    limit: usize,
    explain: bool,
) -> Result<Vec<SymbolSearchResult>, QueryError>

Conceptual full-text search that also attaches the rerank breakdown to every row when explain is true. Without it, explain stays None on every row.