Fix ProxyBuilder to correctly handle read-only Map properties with @NamedArg constructor

[stechy1]

ProxyBuilder is used by the FXML loader to instantiate classes whose constructors are annotated with @NamedArg. When such a class also exposes a read-only Map property (e.g. getProperties() — the same pattern used by javafx.scene.Node), setting child elements under that property in FXML caused incorrect behaviour or a runtime error.

Root cause: getReadOnlyProperty() always returned an ArrayListWrapper regardless of the actual getter return type. When the getter returns a Map, an ArrayListWrapper is the wrong container and the entries are never transferred to the real map on the object.

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[stechy1]

/signed

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