When a member mapping referenced a mapping by name (Use = ...) that is a new instance mapping, and the target member was a read-only collection or dictionary (e.g. a protobuf MapField), Mapperly silently ignored the configuration and built an implicit existing target mapping instead, producing a confusing RMG020 diagnostic.

The referenced new instance mapping is now respected: it is assigned for settable members and reports RMG009 (cannot map to read only member) when the target cannot be set, instead of being silently ignored.

Closes #2352

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