feat: generic struct monomorphization + tkUnknown fix

- Add generic struct instantiation (Box<int>, Pair<T,U>) in HIR lowering
- Fix tkUnknown/tkNamed/tkTypeParam ambiguity in sema.nim (qualify with TypeKind)
- Add generics_struct example
- Update Makefile with new example

Makefile

@@ -3,7 +3,7 @@ SRC := src/main.nim
 OUT := buxc
 BUILD_DIR := build
 
-EXAMPLES := hello fibonacci factorial structs enums methods algebraic_enums generics pattern_matching strings map result_option try_operator
+EXAMPLES := hello fibonacci factorial structs enums methods algebraic_enums generics pattern_matching strings map result_option try_operator generics_struct
 
 .PHONY: all build dev test clean test-examples