fix: dispatch unwrapped optional-closure call g!() through call_closure (issue 0170)

Calling through an unwrapped optional closure (g!()) crashed with LLVM
'Called function must be a pointer!': the indirect-call catch-all else
arm emitted call_indirect on the whole {fn,env} closure struct with a
hardcoded .i64 return. The else arm now inspects inferExprType(callee):
a .closure callee dispatches through call_closure (threads env + ctx via
the [ctx, env, user_args] ABI, returns closure.ret); a plain fn pointer
uses call_indirect with the callee's real function.ret instead of i64.

The filed repro's ?(() -> void) spelling is a tuple-optional (now
diagnosed by the 0165 fix); the real ?Closure(...) layout was already
correct. Verified load-bearing (HEAD crashes) by 3 adversarial reviews,
suite 785/0. Regression: examples/closures/0311-closures-optional-closure.sx.
Filed adjacent bug 0177 (array-element closure direct call crashes).

examples/closures/expected/0311-closures-optional-closure.exit

@@ -0,0 +1 @@
+0

examples/closures/expected/0311-closures-optional-closure.stderr

@@ -0,0 +1 @@
+

examples/closures/expected/0311-closures-optional-closure.stdout

@@ -0,0 +1,14 @@
+a-present
+a n=7
+b-called
+c-absent
+c==null: true
+c!=null: false
+c-reassigned
+c2-absent
+fallback
+e-real
+a n=7
+param-absent
+from-give
+add: 7