fix: restrict inside_cast_in_heap to interface types only

The original fix set inside_cast_in_heap for both sum types and
interfaces when packing values into arrays/variadic args. However,
setting it for sum types caused the compiler to fail self-compilation:
struct init codegen in struct.v line 100 uses inside_cast_in_heap > 0
to add memdup wrapping, which breaks for sum type arrays like
[]ast.Expr used pervasively in the compiler itself.

Restrict to .interface only since:
- The regression test (#26760) only involves interface values
- Interface casts use HEAP() macro (safe) vs & (dangling pointer risk)
- Sum type casts use ADDR() macro which already calls memdup

Fixes self-compilation failure while preserving the variadic interface
chain forwarding fix.

Author: vincenzopalazzo

vlib/v/gen/c/array.v

@@ -59,7 +59,8 @@ fn (mut g Gen) array_init(node ast.ArrayInit, var_name string) {
 			g.write('\t\t')
 		}
 		is_iface_or_sumtype := elem_sym.kind in [.sum_type, .interface]
-		if is_iface_or_sumtype {
+		is_iface := elem_sym.kind == .interface
+		if is_iface {
 			g.inside_cast_in_heap++
 		}
 		for i, expr in node.exprs {
@@ -94,7 +95,7 @@ fn (mut g Gen) array_init(node ast.ArrayInit, var_name string) {
 				}
 			}
 		}
-		if is_iface_or_sumtype {
+		if is_iface {
 			g.inside_cast_in_heap--
 		}
 		g.write('}))')

vlib/v/gen/c/fn.v

@@ -2863,8 +2863,8 @@ fn (mut g Gen) call_args(node ast.CallExpr) {
 							g.write('builtin___option_ok((${base_type}[]) {')
 						}
 						elem_sym := g.table.sym(arr_info.elem_type)
-						is_iface_or_sumtype := elem_sym.kind in [.sum_type, .interface]
-						if is_iface_or_sumtype {
+						is_iface := elem_sym.kind == .interface
+						if is_iface {
 							g.inside_cast_in_heap++
 						}
 						g.write('builtin__new_array_from_c_array${noscan}(${variadic_count}, ${variadic_count}, sizeof(${elem_type}), _MOV((${elem_type}[${variadic_count}]){')
@@ -2876,7 +2876,7 @@ fn (mut g Gen) call_args(node ast.CallExpr) {
 							}
 						}
 						g.write('}))')
-						if is_iface_or_sumtype {
+						if is_iface {
 							g.inside_cast_in_heap--
 						}
 						if is_option {