ERR/E5.1: reject closure-value into bare function-pointer slot

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.

examples/expected/1045-errors-closure-var-bare-slot-reject.exit

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+1

examples/expected/1045-errors-closure-var-bare-slot-reject.stderr

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+error: a closure value cannot be passed as a bare function-pointer `(...) -> ...` — its environment can't be carried across the bare ABI; pass the closure literal directly at the call site, or declare the parameter type as `Closure(...)`
+  --> /Users/agra/projects/sx/examples/1045-errors-closure-var-bare-slot-reject.sx:23:10
+   |
+23 |     _ := bare(inc, 9);          // reject: closure value → bare slot
+   |          ^^^^^^^^^^^^
+
+error: a closure value cannot be passed as a bare function-pointer `(...) -> ...` — its environment can't be carried across the bare ABI; pass the closure literal directly at the call site, or declare the parameter type as `Closure(...)`
+  --> /Users/agra/projects/sx/examples/1045-errors-closure-var-bare-slot-reject.sx:24:10
+   |
+24 |     _ := baref(inc, 9);         // reject: also the ∅-widening crossing
+   |          ^^^^^^^^^^^^^
+
+error: a closure value cannot be passed as a bare function-pointer `(...) -> ...` — its environment can't be carried across the bare ABI; pass the closure literal directly at the call site, or declare the parameter type as `Closure(...)`
+  --> /Users/agra/projects/sx/examples/1045-errors-closure-var-bare-slot-reject.sx:25:10
+   |
+25 |     _ := bare(add, 9);          // reject: capturing closure → bare slot
+   |          ^^^^^^^^^^^^

examples/expected/1045-errors-closure-var-bare-slot-reject.stdout

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+