feat: struct and tuple patterns in match (bootstrap + selfhost)

Support destructuring in match arms:
- Tuple: (a, b) binds subject._0 / _1
- Struct: Point { x: px, y: py } and shorthand Point { x, y }

Bootstrap: matchPatternBindings for pkTuple/pkStruct; register local
tuple typedefs from function bodies. Selfhost: parse, Sema_BindPattern,
Lcx_PatternBindings with scope defines. Fix operator-overload path that
crashed when typeName was null after pattern binds.

Example: examples/struct_tuple_pat.bux. Selfhost-loop IDENTICAL.

src/ast.bux

@@ -76,10 +76,11 @@ struct Pattern {
     patLitText: String,      // for pkLiteral (token text)
     patRangeInclusive: bool, // for pkRange
     patEnumPath: String,     // for pkEnum: "Enum::Variant"
-    patStructName: String,   // for pkStruct
+    patStructName: String,   // for pkStruct (type name)
+    patFieldName: String,    // for struct field entry: field name in Point { x: a }
     patChild1: *Pattern,     // range lo / nested
     patChild2: *Pattern,     // range hi / nested
-    patArgs: *Pattern,       // pkEnum payload args (head)
+    patArgs: *Pattern,       // pkEnum/pkTuple/pkStruct field list (head)
     patNext: *Pattern,       // next sibling in patArgs list
 }