Decompile ov04 (field gymmick)

[PikalaxALT]

Must come after #495

PR checklist

• This comment contains a description of changes (with reason).
• The ROM compiles to matching locally (make compare_heartgold && make compare_soulsilver).
• All C code is correctly formatted (git clang-format).
• This pull request is labeled according to the kind of work it represents.
• New or updated code labels follow the style guide
• This work adheres to the AI policy.
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