fix: resolve closure/fn-pointer struct-field call types (issue 0201)

Calling a closure or function-pointer value stored in a struct data field
(`box.run(args)`) typed the call as 'unresolved': value returns marshalled
as garbage, failable fields could not be try/catch-ed. Lowering already
dispatched these (call_closure / call_indirect); only CallResolver.plan
lacked a field-access arm. Add a closure/fn-pointer field arm to plan
(before the instance-method check, mirroring lowering's precedence — a
closure-typed field shadows a same-named method) and extend the lowering
closure-field arm to also handle bare .function fields via call_indirect.

Lock: examples/closures/0315-closures-struct-field-call.sx.

examples/closures/expected/0315-closures-struct-field-call.stderr

@@ -0,0 +1 @@
+