chore: remove dead UnionAsMapOptions fields

[lalinsky]

Fixes #36.

pub const UnionAsMapOptions = struct {
    key: union(enum) { field_name, field_name_prefix: u8, field_index },
    omit_nulls: bool = true,        // never read
    omit_defaults: bool = false,    // never read
};

Both are copy-paste from StructAsMapOptions, where they are genuinely used by isStructFieldUsed on the encode side. Here nothing consults them in either direction.

They are also meaningless in this position: packUnionAsMap always writes packMapHeader(writer, 1) — one entry, the active variant — so there is nothing to omit.

Same category as the dead surface removed in #27. No behaviour change, since nothing read them.

Breaking only for code that set them explicitly, which could only ever have been a no-op. Recorded in the changelog under Removed.

zig build test: 183/183 pass.

Noticed while adding skip_unknown_fields in #28 and deferred to keep that PR focused.

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Reviewing files that changed from the base of the PR and between db7dc86 and 1881559.

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• CHANGELOG.md
• src/union.zig

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