feat: nested multi-field enum patterns (bootstrap + selfhost)

Multi-field variants use Enum_Variant_Payload nested types (avoids tag name clash).
Sema resolves data.Variant.Variant_i; enum field types fully resolve tuples.
Selfhost parses full type exprs in enum payloads; emit structs/tuples before enums.
Example: examples/nested_patterns.bux (Pair::Two, Box::Val((a,c)), Shape::Dot).

src/hir.bux

@@ -126,8 +126,10 @@ struct HirEnumVariant {
     fieldCount: int;
     fieldType0: int;
     fieldName0: String;
+    fieldTypeName0: String;  // C type name (e.g. "int", "Tuple_int_int")
     fieldType1: int;
     fieldName1: String;
+    fieldTypeName1: String;
 }
 
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