t27b frontend: assigning i32 to ?i32 is rejected; the reference passes it (gen_w378.t27) #6186

Refs #6063

Lab run dd3864d (master, 2026-10-04T16:06Z): t27b's frontend rejects specs/port/scripts/gen_w378.t27, and the reference path (t27c gen + zig) passes it.

t27b: typecheck error in specs/port/scripts/gen_w378.t27: type mismatch at line 44: cannot assign I32 to Custom("?i32")

Line 43 is wave = wave_num;, where wave is declared ?i32 and wave_num is i32. Zig coerces T to ?T implicitly, so the reference is right to accept it.

mismatch does not count this kind of disagreement, because the spec never reaches codegen. From now on tri t27b doctor reports it as LAB-FRONTEND-DISAGREES (#6185).

Done when: assigning T to ?T typechecks in t27b's frontend; the spec passes on the lab or reaches a real codegen blocker; and a unit test covers the coercion.