add enum_min and enum_max

[aouxwoux]

what changed

• add enum_min and enum_max to the public api
• support the default reflected subtype and explicit �s_flags<>
• add common/flags tests and reference docs

why

detail::min_v and detail::max_v are useful when mapping reflected enum values, but they are internal. this exposes the same information through the existing function-based api while keeping those implementation details private.

the functions return the first and last reflected values, so custom ranges and flags use the same ordering as enum_values.

addresses #462

checks

• git diff --check
• not run locally: this environment does not have a c++ compiler/toolchain