TypeInfo gains a `struct(StructInfo) variant (StructField{name,type});
the metatype system now reflects AND constructs structs, not just enums.
- meta.sx: StructField / StructInfo / `struct TypeInfo variant.
- interp: reflectTypeInfo builds .struct (tag 1) for a source @"struct";
define dispatches on the TypeInfo tag (defineType) -> defineEnum (0) /
defineStruct (1). defineStruct mirrors defineEnum (dup-field-name check
included) but completes the declare slot AS a struct via replaceKeyedInfo
(a kind change re-keys the intern map; updatePreservingKey asserts no
key change, true only for the enum path).
- call.zig: the lower-time type_info guard now admits @"struct".
define(declare("P"), .struct(.{ fields = .[ … ] })) builds a struct, and
define(declare("C"), type_info(SrcStruct)) round-trips one. Suite green
(682); enum path (0619) unchanged.
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