[PATCH] Fix Box regexp match vars after non-match

	[PATCH] Use box_ready for $&, $`, $\', $+

	These variables have rb_gvar_readonly_setter, so box_ready is sufficient.
	Only $~ needs box_dynamic due to its custom match_setter.

Using `assume_single_ractor_mode` we can skip all ractor safety
checks if we're in single ractor mode.

```
compare-ruby: ruby 3.5.0dev (2025-08-27T14:58:58Z merge-vm-setivar-d.. 5b749d8e53) +YJIT +PRISM [arm64-darwin24]
built-ruby: ruby 3.5.0dev (2025-08-28T21:23:38Z yjit-get-exivar 3cc21b76d4) +YJIT +PRISM [arm64-darwin24]

|                           |compare-ruby|built-ruby|
|:--------------------------|-----------:|---------:|
|vm_ivar_get_on_obj         |     975.981|   975.772|
|                           |       1.00x|         -|
|vm_ivar_get_on_class       |     136.214|   470.912|
|                           |           -|     3.46x|
|vm_ivar_get_on_generic     |     148.315|   299.122|
|                           |           -|     2.02x|
```

While accessing the ivars of other types is too complicated to
realistically generate the ASM for it, we can at least provide
the ivar index as to not have to lookup the shape tree every
time.

```
compare-ruby: ruby 3.5.0dev (2025-08-27T14:58:58Z merge-vm-setivar-d.. 5b749d8e53) +YJIT +PRISM [arm64-darwin24]
built-ruby: ruby 3.5.0dev (2025-08-28T17:58:32Z yjit-get-exivar efaa8c9b09) +YJIT +PRISM [arm64-darwin24]

|                           |compare-ruby|built-ruby|
|:--------------------------|-----------:|---------:|
|vm_ivar_get_on_obj         |     930.458|   936.865|
|                           |           -|     1.01x|
|vm_ivar_get_on_class       |     134.471|   431.622|
|                           |           -|     3.21x|
|vm_ivar_get_on_generic     |     146.679|   284.408|
|                           |           -|     1.94x|
```

Co-Authored-By: Aaron Patterson <tenderlove@ruby-lang.org>

`vm_setinstancevariable` had a codepath to try to match the inline
cache for types other than T_OBJECT, but the cache population path
in `vm_setivar_slowpath` was exclusive to T_OBJECT, so `vm_setivar_default`
would never match anything.

This commit improves `vm_setivar_slowpath` so that it is capable of
filling the cache for all types, and adds a `vm_setivar_class` codepath
for `T_CLASS` and `T_MODULE`.

`vm_setivar`, `vm_setivar_default` and `vm_setivar_class` could be unified,
but based on the very explicit `NOINLINE` I assume they were split to minimize
codesize.

```
compare-ruby: ruby 3.5.0dev (2025-08-27T14:58:58Z merge-vm-setivar-d.. 5b749d8e53) +PRISM [arm64-darwin24]
built-ruby: ruby 3.5.0dev (2025-08-27T16:30:31Z setivar-cache-gene.. 4fe78ff296) +PRISM [arm64-darwin24]

|                         |compare-ruby|built-ruby|
|:------------------------|-----------:|---------:|
|vm_ivar_set_on_instance  |     161.809|   164.688|
|                         |           -|     1.02x|
|vm_ivar_set_on_generic   |      58.769|   115.638|
|                         |           -|     1.97x|
|vm_ivar_set_on_class     |      70.034|   141.042|
|                         |           -|     2.01x|
```

All accesses to `generic_fields_tbl_` are now encapsulated inside:

  - `rb_obj_fields`
  - `rb_obj_set_fields`
  - `rb_obj_replace_fields`

These two functions are very similar, they can share most of their
logic.

Followup: https://github.com/ruby/ruby/pull/13589

This simplify a lot of things, as we no longer need to manually
manage the memory, we can use the Read-Copy-Update pattern and
avoid numerous race conditions.

Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>

The type isn't opaque because Ruby isn't often compiled with LTO,
so for optimization purpose it's better to allow as much inlining
as possible.

However ideally only `shape.c` and `shape.h` should deal with
the actual struct, and everything else should just deal with opaque
`shape_id_t`.