t27b ready: a red non-required check against a still-running master tip must use master's last completed verdict or WAIT #6334

Refs #6063

Problem

tri t27b ready / watch let #6333 merge at 02:02Z with coverage=FAILURE (master PENDING). Master's tip run of coverage was only queued, so the comparison proved nothing (it happened to be safe: coverage failed on the 3 previous master commits).

Two causes:

1. pr_ready in specs/tri/t27b/steward.t27 folds the non-required effects but only acts on BLOCK (other == 2); a WAIT from a non-required check (check_effect(2, 1, false) == 1, Q29) fell through to READY.
2. fold_master reports PENDING as soon as the tip's run is going, hiding the older completed verdict, so the report cannot say what master actually was.

Fix

• steward.t27: pr_ready waits on a non-required WAIT; new master_state(running, last) decides which master state a check is judged against: a red verdict under a running tip stays red (master was already broken), a green one is PENDING (Q29), no verdict in the window is PENDING (WAIT, never MERGE).
• t27b.py (plumbing): fold_master keeps the newest completed verdict and its sha plus whether a newer run is still going; ready reports e.g. coverage=FAILURE (master FAILURE@f329e27c1).
• gen/c/tri/t27b/steward.c regenerated; fixture tests in scripts/ci.

Acceptance

• tri t27b ready on a fixture with a red PR check and master {running, last FAILURE@sha} says READY with (master FAILURE@sha...); with {running, no verdict} says WAIT.
• spec tests pass in the generated C; tri t27b gen-check says SAME.