encoding.cbor: build option payloads via comptime $zero(field.typ.payload_type)

The generic struct decoder recovered an option field's payload type through a
helper whose only job was to abuse generic inference:

    fn create_value_from_optional[T](_val ?T) T { return T{} }

#27048 makes `$zero(TypeExpr)` resolve per-field comptime metadata, so the
payload type can be read directly at the `$for field` site. Replace the call with
`$zero(field.typ.payload_type)` and drop the helper.

No behavior change: the helper unconditionally returned `T{}` (zero of the payload),
exactly what `$zero(field.typ.payload_type)` yields; `inner` is then filled by
`unpack_into`. The full `vlib/encoding/cbor` suite (10 files) still passes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: enghitalo

vlib/encoding/cbor/generic.v

@@ -416,7 +416,7 @@ fn (mut u Unpacker) unpack_struct_into[T](mut result T) ! {
 							u.pos++
 							result.$(field.name) = none
 						} else {
-							mut inner := create_value_from_optional(result.$(field.name))
+							mut inner := $zero(field.typ.payload_type)
 							u.unpack_into(mut inner)!
 							result.$(field.name) = inner
 						}
@@ -570,12 +570,6 @@ fn utf8_validate_slice(data []u8, start int, size int) bool {
 	return true
 }
 
-// create_value_from_optional returns a zero value of an Option's inner T.
-// Exists so the comptime call site can infer T from a struct field.
-fn create_value_from_optional[T](_val ?T) T {
-	return T{}
-}
-
 // unpack_into fills the target through a mutable reference. The mut
 // parameter exists so V's generic inferer picks up T from the
 // `u.unpack_into(mut result.$(field.name))!` call site.