Fix id2ref for multi-Ractor

The id2ref table needs to be under a VM lock to ensure there are no race
conditions. The following script crashes:

    o = Object.new

    ObjectSpace._id2ref(o.object_id)

    10.times.map do
      Ractor.new do
        10_000.times do
          a = Object.new
          a.object_id
        end
      end
    end.map(&:value)

With:

    [BUG] Object ID seen, but not in _id2ref table: object_id=2800 object=T_OBJECT
    ruby 4.0.0dev (2025-12-06T15:15:43Z ractor-id2ref-fix e7f9abd) +PRISM [x86_64-linux]

    -- Control frame information -----------------------------------------------
    c:0001 p:---- s:0003 e:000002 l:y b:---- DUMMY  [FINISH]

    -- Threading information ---------------------------------------------------
    Total ractor count: 5
    Ruby thread count for this ractor: 1

    -- C level backtrace information -------------------------------------------
    miniruby(rb_print_backtrace+0x14) [0x6047d09b2dff] vm_dump.c:1105
    miniruby(rb_vm_bugreport) vm_dump.c:1450
    miniruby(rb_bug_without_die_internal+0x5f) [0x6047d066bf57] error.c:1098
    miniruby(rb_bug) error.c:1116
    miniruby(rb_gc_get_ractor_newobj_cache+0x0) [0x6047d066c8dd] gc.c:2052
    miniruby(gc_sweep_plane+0xad) [0x6047d079276d] gc/default/default.c:3513
    miniruby(gc_sweep_page) gc/default/default.c:3605
    miniruby(gc_sweep_step) gc/default/default.c:3886
    miniruby(gc_sweep+0x1ba) [0x6047d0794cfa] gc/default/default.c:4154
    miniruby(gc_start+0xbf2) [0x6047d0796742] gc/default/default.c:6519
    miniruby(heap_prepare+0xcc) [0x6047d079748c] gc/default/default.c:2090
    miniruby(heap_next_free_page) gc/default/default.c:2305
    miniruby(newobj_cache_miss) gc/default/default.c:2412
    miniruby(newobj_alloc+0xd) [0x6047d0798ff5] gc/default/default.c:2436
    miniruby(rb_gc_impl_new_obj) gc/default/default.c:2515
    miniruby(newobj_of) gc.c:996
    miniruby(rb_wb_protected_newobj_of) gc.c:1046
    miniruby(str_alloc_embed+0x28) [0x6047d08fda18] string.c:1019
    miniruby(str_enc_new) string.c:1069
    miniruby(prep_io+0x5) [0x6047d07cda14] io.c:9305
    miniruby(prep_stdio) io.c:9347
    miniruby(rb_io_prep_stdin) io.c:9365
    miniruby(thread_start_func_2+0x77c) [0x6047d093a55c] thread.c:679
    miniruby(thread_sched_lock_+0x0) [0x6047d093aacd] thread_pthread.c:2241
    miniruby(co_start) thread_pthread_mn.c:469

Author: peterzhu2118

bootstraptest/test_ractor.rb

@@ -1116,6 +1116,23 @@ class C
   end.value
 RUBY
 
+# Inserting into the id2ref table should be Ractor-safe
+assert_equal 'ok', <<~'RUBY'
+  # Force all calls to Kernel#object_id to insert into the id2ref table
+  ObjectSpace._id2ref(Object.new.object_id)
+
+  10.times.map do
+    Ractor.new do
+      10_000.times do
+        a = Object.new
+        a.object_id
+      end
+    end
+  end.map(&:value)
+
+  :ok
+RUBY
+
 # Ractor.make_shareable(obj)
 assert_equal 'true', <<~'RUBY', frozen_string_literal: false
   class C

gc.c

@@ -1905,7 +1905,9 @@ object_id0(VALUE obj)
     RUBY_ASSERT(rb_shape_obj_has_id(obj));
 
     if (RB_UNLIKELY(id2ref_tbl)) {
-        st_insert(id2ref_tbl, (st_data_t)id, (st_data_t)obj);
+        RB_VM_LOCKING() {
+            st_insert(id2ref_tbl, (st_data_t)id, (st_data_t)obj);
+        }
     }
     return id;
 }