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These are almost the same as the 4.0.0 headers, yet the change to add
`is_ready` in `rb_thread_sched_item` shifted the in-memory layouts of
things inside `rb_thread_struct` which broke the profiler.

```diff
$ diff -uw ruby-4.0.0-p0/ ruby-4.0.6-p0/
Common subdirectories: ruby-4.0.0-p0/ccan and ruby-4.0.6-p0/ccan
diff -uw ruby-4.0.0-p0/id_table.h ruby-4.0.6-p0/id_table.h
--- ruby-4.0.0-p0/id_table.h	2026-01-07 15:43:22.492338852 +0000
+++ ruby-4.0.6-p0/id_table.h	2026-07-15 09:07:09.200855897 +0100
@@ -47,6 +47,16 @@

 extern const rb_data_type_t rb_managed_id_table_type;

+VALUE rb_marked_id_table_new(size_t capa);
+int rb_marked_id_table_insert(VALUE table, ID id, VALUE val);
+VALUE rb_marked_id_table_dup(VALUE table);
+
+// alisases
+#define rb_marked_id_table_size             rb_managed_id_table_size
+#define rb_marked_id_table_lookup           rb_managed_id_table_lookup
+#define rb_marked_id_table_foreach          rb_managed_id_table_foreach
+#define rb_marked_id_table_foreach_values   rb_managed_id_table_foreach_values
+
 RUBY_SYMBOL_EXPORT_BEGIN
 size_t rb_id_table_size(const struct rb_id_table *tbl);
 RUBY_SYMBOL_EXPORT_END
Common subdirectories: ruby-4.0.0-p0/internal and ruby-4.0.6-p0/internal
Common subdirectories: ruby-4.0.0-p0/prism and ruby-4.0.6-p0/prism
diff -uw ruby-4.0.0-p0/thread_pthread.h ruby-4.0.6-p0/thread_pthread.h
--- ruby-4.0.0-p0/thread_pthread.h	2026-01-07 15:43:22.497873080 +0000
+++ ruby-4.0.6-p0/thread_pthread.h	2026-07-15 09:07:09.202007892 +0100
@@ -56,6 +56,9 @@
         // connected to ractor->threads.sched.reqdyq
         // locked by ractor->threads.sched.lock
         struct ccan_list_node readyq;
+        // Indicates whether thread is on the readyq.
+        // There is no clear relationship between this and th->status.
+        bool is_ready;

         // connected to vm->ractor.sched.timeslice_threads
         // locked by vm->ractor.sched.lock
@@ -127,6 +130,11 @@
     struct rb_thread_struct *lock_owner;
 #endif
     struct rb_thread_struct *running; // running thread or NULL
+    // Most recently running thread or NULL. If this thread wakes up before the newly running
+    // thread completes the transfer of control, it can interrupt and resume running.
+    // The new thread clears this field when it takes control.
+    struct rb_thread_struct *runnable_hot_th;
+    int runnable_hot_th_waiting;
     bool is_running;
     bool is_running_timeslice;
     bool enable_mn_threads;
diff -uw ruby-4.0.0-p0/vm_core.h ruby-4.0.6-p0/vm_core.h
--- ruby-4.0.0-p0/vm_core.h	2026-01-07 15:43:22.497873080 +0000
+++ ruby-4.0.6-p0/vm_core.h	2026-07-15 09:07:09.202386196 +0100
@@ -763,6 +763,7 @@
     const VALUE special_exceptions[ruby_special_error_count];

     /* Ruby Box */
+    rb_box_t *master_box;
     rb_box_t *root_box;
     rb_box_t *main_box;

@@ -2233,6 +2234,7 @@
 int rb_signal_exec(rb_thread_t *th, int sig);
 void rb_threadptr_check_signal(rb_thread_t *mth);
 void rb_threadptr_signal_raise(rb_thread_t *th, int sig);
+void rb_threadptr_interrupt_raise(rb_thread_t *th);
 void rb_threadptr_signal_exit(rb_thread_t *th);
 int rb_threadptr_execute_interrupts(rb_thread_t *, int);
 void rb_threadptr_interrupt(rb_thread_t *th);
```

…port

[PROF-15412] Import Ruby 4.0.6 headers and bump gem version to 3.5.3