When mrb_mt_init_rom() is called on a class that already has a
mutable method table (from prior mrb_define_method_id() calls),
the mutable top layer is now frozen in place instead of being
left as a writable layer that wastes RAM on embedded systems.
The frozen bit (bit 29 of alloc field) marks heap-allocated
method table layers as temporarily immutable. Unlike the
readonly bit (bit 30, for true ROM), frozen layers are
automatically unfrozen when methods are later added via
mrb_define_method_raw() or removed via mrb_remove_method().
This preserves the c->mt pointer, which is critical because
iclasses (from module inclusion) hold a copy of it.
Co-authored-by: Claude <noreply@anthropic.com>
Yukihiro "Matz" Matsumoto
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