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.

src/ir/calls.zig

@@ -270,6 +270,47 @@ pub const CallResolver = struct {
                     }
                 }
             }
+            // Struct field holding a CLOSURE value, called directly
+            // (`box.run(args)` where `run: Closure(..) -> R`). Mirrors the
+            // lowering dispatch (call.zig closure-field arm) which runs in the
+            // value-receiver path BEFORE instance-method dispatch — so a
+            // closure-typed field shadows a same-named method, exactly as
+            // lowering binds it. Without this the call typed as `.unresolved`
+            // (issue 0201): value returns marshalled as garbage, failable
+            // returns couldn't be `try`/`catch`-ed. Lowering owns the dispatch;
+            // plan only needs the field's `.ret` so typing matches.
+            {
+                var fld_recv = recv_ty;
+                if (!fld_recv.isBuiltin()) {
+                    const ri = self.l.module.types.get(fld_recv);
+                    if (ri == .pointer) fld_recv = ri.pointer.pointee;
+                }
+                if (!fld_recv.isBuiltin()) {
+                    const ri = self.l.module.types.get(fld_recv);
+                    if (ri == .@"struct") {
+                        const field_name_id = self.l.module.types.internString(cfa.field);
+                        for (ri.@"struct".fields) |f| {
+                            if (f.name == field_name_id and !f.ty.isBuiltin()) {
+                                const fti = self.l.module.types.get(f.ty);
+                                if (fti == .closure) return .{
+                                    .kind = .closure,
+                                    .return_type = fti.closure.ret,
+                                    .target = .{ .named = cfa.field },
+                                };
+                                // Bare function-pointer field (`fp: (T) -> R`),
+                                // symmetric with the bare-identifier fn-pointer
+                                // path above — call via `call_indirect`.
+                                if (fti == .function) return .{
+                                    .kind = .fn_pointer,
+                                    .return_type = fti.function.ret,
+                                    .target = .{ .named = cfa.field },
+                                    .prepends_ctx = self.l.implicit_ctx_enabled and fti.function.call_conv != .c,
+                                };
+                            }
+                        }
+                    }
+                }
+            }
             // Instance method call: obj.method(args) → StructName.method.
             {
                 var obj_ty = recv_ty;