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/1045-errors-closure-var-bare-slot-reject.sx

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+// A closure VALUE (a pre-bound variable) cannot be passed into a bare
+// function-pointer slot `(...) -> ...` (ERR E5.1). The bare ABI calls
+// `fn_ptr(ctx, args)` with no env channel, so a closure's environment can't be
+// carried — passing one is unsound (drops env / shifts args / segfaults on a
+// capturing closure). Only a closure LITERAL crosses this boundary (lowerLambda
+// emits a static adapter); a variable is rejected with a pointer to the idiom.
+//
+// The fix for these is either to pass the literal directly, or to type the
+// parameter `Closure(...)` so the environment is carried (the idiomatic form).
+
+#import "modules/std.sx";
+
+E :: error { Z }
+
+bare  :: (cb: (s64) -> s64, n: s64) -> s64 { return cb(n); }
+baref :: (cb: (s64) -> (s64, !E), n: s64) -> s64 { return cb(n) catch e -1; }
+
+main :: () -> s32 {
+    inc := closure((x: s64) -> s64 => x + 1);          // capture-free closure var
+    base := 100;
+    add := closure((x: s64) -> s64 => x + base);       // CAPTURING closure var
+
+    _ := bare(inc, 9);          // reject: closure value → bare slot
+    _ := baref(inc, 9);         // reject: also the ∅-widening crossing
+    _ := bare(add, 9);          // reject: capturing closure → bare slot
+    return 0;
+}

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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+