feat(selfhost): generic monomorphization + collection-based for-in loops

- Add on-demand generic function/struct monomorphization in HIR lower
  - Lcx_SubstituteType: substitute type params and mangle names (e.g. Array<int> -> Array_int)
  - Lcx_GenerateFuncInstance: clone generic func with active substitution
  - Lcx_GenerateStructInstance: create concrete struct definitions
  - Detect generic calls (ekCall) and struct inits (ekStructInit)

- Parser: propagate generic type args from ident to ekStructInit node

- Fix duplicate HIR constant: hIndexPtr and hCall both had value 18,
  causing index expressions to emit as comma expressions in C backend

- Implement collection-based for-in desugaring:
  for x in arr { body } -> __iter = Array_Iter_T(&arr);
                           while Iter_HasNext_T(&__iter) { x = Iter_Next_T(&__iter); body }

Selfhost loop remains deterministic. All golden tests and examples pass.

src/parser.bux

@@ -678,6 +678,10 @@ func parserParsePostfixExpr(p: *Parser) -> *Expr {
                 e.structName = typeName;
                 e.structFieldCount = fieldCount;
                 e.child1 = firstField;
+                // Propagate generic type args from the identifier
+                e.genericTypeArg0 = left.genericTypeArg0;
+                e.genericTypeArg1 = left.genericTypeArg1;
+                e.genericTypeArgCount = left.genericTypeArgCount;
                 left = e;
                 continue;
             } else {