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.

src/ir/lower/call.zig

@@ -1731,16 +1731,15 @@ pub fn tryLowerReflectionCall(self: *Lowering, name: []const u8, c: *const ast.C
             return Ref.none;
         };
         const t = self.resolveTypeArg(c.args[0]);
-        // Only enum / tagged-union reflection ships today (the symmetric
-        // inverse of `define`, which builds tagged-unions). A loud, well-
-        // spanned reject here beats a deferred interp bail; struct/tuple
-        // widening lands later.
+        // Enum / tagged-union and struct reflection ship today (the symmetric
+        // inverse of `define`, which builds those kinds). A loud, well-spanned
+        // reject here beats a deferred interp bail; tuple widening lands later.
         const reflectable = !t.isBuiltin() and switch (self.module.types.get(t)) {
-            .@"enum", .tagged_union => true,
+            .@"enum", .tagged_union, .@"struct" => true,
             else => false,
         };
         if (!reflectable) {
-            if (self.diagnostics) |d| d.addFmt(.err, c.args[0].span, "type_info: '{s}' is not an enum — only enum/tagged-union reflection is supported today", .{self.formatTypeName(t)});
+            if (self.diagnostics) |d| d.addFmt(.err, c.args[0].span, "type_info: '{s}' is not reflectable — only enum / tagged-union / struct types reflect today", .{self.formatTypeName(t)});
             return Ref.none;
         }
         const type_ref = self.builder.constType(t);