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Typed Python §4: show the Any of is_pair and is_list as object (no more LList[Any] in messages) (#657)

* Typed Python §4: is_pair narrows an object to Pair[object, object], not Pair[Any, Any]

is_pair has an overload for a pair or None, which keeps its types, and one for an object. After the
second, head(x) and tail(x) are objects, and the message for arity(tail(x)) no longer says LList[Any].
is_list keeps Any in its TypeIs.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

* Typed Python §4: is_list narrows to Sequence[object], shown as list[object]; no Any in the prelude

list[object] would not keep a list[int] when a union is narrowed (list is invariant): int | list[int]
was rejected. Sequence is covariant. The messages call it list[object], with two rules for what a
student then tries: assigning to an element and calling append.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

* Typed Python §4: keep Any in the TypeIs of is_pair and is_list, and show it as object

The overloads of is_pair and the Sequence[object] of is_list broke the narrowing of unions: the branch of
is_pair(x) for str | Pair[int, int] was unreachable, and a str counted as a list for str | list[int].
Only Any narrows a union correctly. Messages show Any as the argument of a type as object (LList[object]),
and the documentation prints it as object.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

* Tests: the comment of the is_stream exclusion is as on main

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

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Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

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