…hon#1458)

The previous spec described Never in four lines without defining any of
its subtyping behaviour. Type checkers converge on a consistent set of
rules, but none were written down. This commit codifies them.

Spec changes (docs/spec/special-types.rst):
- Never is a subtype of every fully static type (bottom type property)
- No type other than Never (and Any) is a subtype of Never
- Never | T collapses to T for any type T
- Never as a type argument: covariant containers accept it,
  invariant containers do not
- type[Never] represents an uninhabitable class object
- Clarify that NoReturn may appear in non-return positions

Conformance test changes (conformance/tests/specialtypes_never.py):
- Add func11: Never | int collapses to int
- Add func12: object is not assignable to Never (E)
- Add func13: raise in Never-returning function is OK
- Add func14: type[Never] is a valid annotation
- Add func15: list[Never] return is valid
- Add func16: list[Never] is not assignable to list[int] (E)

Result TOMLs updated for mypy, pyright, pyrefly, zuban (Pass).
ty marked Partial: does not flag func12 and func16 errors.

Closes python#1458

- Use int instead of object as the non-Never witness (object is already
  expected to not be a subtype of most things; int is more illuminating)
- Rephrase to avoid the too-strong 'No type other than Never' claim
  (uninhabitable types like tuple[Never, int] are also subtypes of Never)
- Qualify all 'every type T' as 'every fully static type T' throughout
  (the subtype relation is undefined for gradual types like Any)
- Replace list[Never] with Sequence[Never] in the covariant example
  (list is invariant, so list[Never] is not assignable to list[int];
  Sequence is covariant and correctly demonstrates the bottom-type behaviour)