[d0ra-1h3-3xpl0ra]

Fixes #1063.

IrGenerator never set presence on the BEGIN_ENUM/END_ENUM encoding, so it was always REQUIRED. It is now OPTIONAL when the enum type is optional: either its own presence attribute is optional, or its encodingType names an optional type (e.g. uInt8NULL in the CME iLink schemas). Everything else stays REQUIRED; constant is not propagated.

A field's own presence stays on the BEGIN_FIELD token, where generators already read it. The enum token describes the type, like composite ENCODING tokens, and it is shared through Ir.types(), so it shouldn't depend on which field referenced it. OTF users read the value from the BEGIN_ENUM token (tokens[fromIndex] in onEnum).

Generated code: I generated all 84 valid schemas in the repo before and after, for Java (+DTOs), C++ (+DTOs, +IR), C, C#, Go (struct and flyweight) and Rust, and the sources are byte-identical. Only the .sbeir of the 5 schemas that use an optional enum changes. One case no repo schema covers is an optional enum inside a composite:

• the C++, C and Go flyweight PRESENCE meta attribute becomes "optional";
• the Rust composite encoder gains the <name>_opt setter that RustGenerator already has a branch for. The generated crate compiles.

The Java build and tests pass. I didn't run the C++/C, Go and C# test suites locally.