Do not pass a raw pointer as the recursion key to rb_exec_recursive()

[koic]

The spec added in 20ae94d (for truffleruby/truffleruby#4334) passes a pointer to a 4-byte C stack struct as both the obj and arg arguments of rb_exec_recursive().

While arg is passed through to the callback as-is, obj is used as a key of the recursion guard hash in CRuby. Storing it there fires the GC write barrier, which reads the (8-byte) object flags from the non-object address, and AddressSanitizer reports a stack-buffer-overflow:

==29649==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7f0d9b7c7b20 ...
READ of size 8 at 0x7f0d9b7c7b20 thread T0
    #0 RB_BUILTIN_TYPE include/ruby/internal/value_type.h:191:30
    #1 rb_gc_impl_writebarrier gc/default/default.c:6621:5
    #2 rb_gc_writebarrier gc.c:3433:5
    #3 rb_obj_written include/ruby/internal/gc.h:641:9
    #4 tbl_update hash.c:1726:5
    #5 rb_hash_aset hash.c:2977:9
    #6 exec_recursive thread.c:5602:13
    #7 kernel_spec_rb_exec_recursive_with_pointer spec/ruby/optional/capi/ext/kernel_spec.c:370:10

Address 0x7f0d9b7c7b20 is located in stack of thread T0 at offset 32 in frame
    #0 kernel_spec_rb_exec_recursive_with_pointer kernel_spec.c:368
  This frame has 1 object(s):
    [32, 36) 'data' (line 369) <== Memory access at offset 32 partially overflows this variable

User impact

Since this spec was synced into ruby/ruby (ruby/ruby@0d68ef6b77), the build (ubuntu-24.04, asan) job of the https://github.com/ruby/ruby-dev-builder daily builds has been crashing in optional/capi/kernel_spec.rb, blocking the ruby-head releases used by ruby/setup-ruby since 2026-07-08 (e.g. https://github.com/ruby/ruby-dev-builder/actions/runs/29044853326).

As a result, downstream CIs that use ruby-head via ruby/setup-ruby are stuck with the 2026-07-06 snapshot, which predates ruby/ruby@291d0d95ea ("onig_reg_copy_body: fix copying of exact_end"). This is how the problem was found: the "Spec - ubuntu head" job on the default branch of rubocop/rubocop keeps failing because Regexp.new(regexp) copies made for the AllowedPatterns config silently fail to match on that snapshot:
https://github.com/rubocop/rubocop/actions/runs/29061954887/job/86265559997

Fix

The intent of the spec is that the callback argument is passed as-is, so this change keeps the raw pointer for arg and uses a real Ruby object (self) as the recursion key. The example still passes on CRuby:

$ make test-spec MSPECOPT=spec/ruby/optional/capi/kernel_spec.rb
1 file, 117 examples, 297 expectations, 0 failures, 0 errors, 0 tagged