Add a dedicated uint32_t flags field to mrb_mt_entry instead of
packing flags into the lower bits of mrb_sym via MRB_MT_KEY().
The key field now stores the pure symbol ID with no shift.
On 64-bit, the flags field fills the alignment gap after mrb_sym,
so entry size remains 16 bytes (zero overhead). On 32-bit, entry
size grows from 8 to 12 bytes.
This eliminates the risk of symbol ID overflow from the 4-bit
shift, and the flags field can later store aspec (MRB_ARGS_*)
information that was previously discarded.
Co-authored-by: Claude <noreply@anthropic.com>
Yukihiro "Matz" Matsumoto
ce8ce3f96d