tri t27b next: the reference-backed lane picker (slip Q38 as a tool) #6317

Refs #6063

The ExprBinary-statement lane (#6316) was picked from t27b corpus --blockers without the reference: all 12 of its files fail to compile in the reference, so the lane moved the counted pass number by 0 (steward skill slip Q38).

tri t27b next reads the lab's latest run (or --fixture DIR) and ranks blocker families for the next coverage lane counting ONLY files where reference == pass and t27b == blocked:

• sole: files where the family is the only blocker (unlocks on its own),
• first: files where it is the first blocker,
• the honest denominator: reference passes split into t27b pass / pass_vacuous / blocked / other, and t27b pass over reference passes with tests.

Families whose files fail in the reference are listed separately as reference bugs to file, not lanes.

The decisions (which side a file counts on, the lane score, the with-tests denominator) live in specs/tri/t27b/steward.t27 and reach Python through gen/c/tri/t27b/steward.c (AGENTS.md "t27b is written in t27"). --json too.