A closure VALUE (a pre-bound variable) flowing into a bare (T)->U slot
was passed unsoundly: the bare ABI calls fn_ptr(ctx, args) with no env
channel, so the closure's underlying fn (which takes an env slot) had its
env dropped and args shifted — UB for a matching ABI, a wrong-tuple read
for the non-failable->failable widening (returned -1), and a segfault when
the closure captured.
coerceToType now rejects a .closure -> .function coercion with a
diagnostic pointing at the idiom (pass the literal directly, which gets
the static adapter, or type the parameter Closure(...) so the env is
carried). Closure LITERALS are unaffected — lowerLambda pre-adapts them to
a .function-typed value before coercion.
Regression: 1045-errors-closure-var-bare-slot-reject.sx.
