fn trace_needle(q: &Search) -> Option<[u8; 16]>Expand description
The trace id this search pins down exactly, if it pins one down.
Only trace_id = <32 hex chars> qualifies, on either signal — logs blocks
carry the same filter as span blocks, so “the logs for this trace” prunes
exactly as hard as “the spans for it”. Terms are AND-ed, so one such term is
enough no matter what else is in the list: a block that cannot hold the id
cannot hold a row satisfying the conjunction.