Fix _Atomic compound assignment type error on clang

Clang considers the result type of `E1 op= E2` on _Atomic(size_t) to
remain _Atomic(size_t), which cannot implicitly initialize a plain
size_t. Add an explicit cast.

https://claude.ai/code/session_016WrrtwsGrysn69mESb975G

Author: claude

src/libAtomVM/resources.c

@@ -454,7 +454,7 @@ void resource_type_fire_monitor(struct ResourceType *resource_type, ErlNifEnv *e
     // Balance the increment from enif_monitor_process.
     // Capture the return value of the atomic RMW directly; a separate
     // load would race with concurrent decrements.
-    size_t val = (refc->ref_count -= REFC_MONITOR_INC);
+    size_t val = (size_t) (refc->ref_count -= REFC_MONITOR_INC);
     if ((val & REFC_DYING_FLAG) && !(val & REFC_MONITOR_MASK)) {
         resource_unmark_serialized(refc->data, refc->resource_type);
         synclist_remove(&env->global->refc_binaries, &refc->head);