Simplify JSON and XML de/serialization in Java

[mristin]

We add generic parseArray/serializeArray helpers for JSON list (de)serialization, replacing per-property inlined boilerplate with shared calls.

We factor out a generic serializeElement helper for XML serialization, mirroring C# (#676). It unifies the previously duplicated writeStartElement/topLevel/writeEndElement/ try-catch wrapping across all five property kinds (primitive, enumeration, interface, concrete class, list), which also tightens two branches' overly broad catch (Exception) down to catch (XMLStreamException), matching what the other three branches already did. A further serializeItems helper does the list-item iteration generically as well, so every list-typed property collapses to a single call instead of its own inlined for loop.

Rather than let the per-item and per-property content-writing logic live on as duplicated lambda text at each call site, we follow the shape of the merged C++ simplification (SerializeBool/ SerializeInt64/etc. plus a serialize_{enum} per enumeration): we generate named methods once -- writeStringifiedContent (shared across boolean/long/double/String, since Object.toString() is universal in Java, unlike C++), writeByteArrayContent, and one write{Enum}Content per enumeration and reference them via this::... from both the standalone property and the list-item path.

A per-enum method is still needed there, even though the string conversion itself is shared (see below), because Java resolves that overload statically per concrete enum type, so a single generic wrapper cannot call it.

We add Stringification.mustToString, a per-enum helper that returns the string representation of a literal or throws, and reuse it for JSON's {enum}ToJsonValue and for XML's per-enum content writer, removing three separate copies of the same Optional-or-throw check.

We promote the _Result<T> class -- previously defined independently, and near-identically, in both Jsonization.java and Xmlization.java -- to a single shared Reporting.Result<T>, since Reporting is already imported by both.

Finally, on the JSON side, we factor the byte-array-to-JsonNode conversion (Base64-encode, then wrap in a text node) into a bytesToJsonNode method on _Transformer, mirroring the existing toJsonNode for Long.