feat(metatype): widen type_info/define to struct types

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.

library/modules/std/meta.sx

@@ -23,12 +23,26 @@ EnumInfo :: struct {
     variants: []EnumVariant;
 }
 
+// One field of a constructed struct: a name plus its type.
+StructField :: struct {
+    name: string;
+    type: Type;
+}
+
+// The shape of a struct being reflected or constructed. As with `EnumInfo`, the
+// type's NAME travels in `declare(name)`, not here.
+StructInfo :: struct {
+    fields: []StructField;
+}
+
 // The reflected/constructed type shape. A tagged union over the kinds of type
-// that can be minted. Only `` .`enum `` ships today; struct/tuple land later.
-// The variant uses the backtick raw-identifier escape so it reads as the
-// keyword `enum` (`` .`enum(...) ``) rather than a mangled `enum_`.
+// that can be minted — `` .`enum `` and `` .`struct `` ship today (tuple later).
+// The variants use the backtick raw-identifier escape so they read as the
+// keywords `enum` / `struct` (`` .`enum(...) `` / `` .`struct(...) ``) rather than
+// mangled `enum_` / `struct_`.
 TypeInfo :: enum {
     `enum: EnumInfo;
+    `struct: StructInfo;
 }
 
 // The compiler's ONLY type-construction primitives (comptime-only #builtins):