[ty] Avoid hangs on recursive tuple aliases in positive intersections

[charliermarsh]

Summary

Builds on #29057. We now handle recursive tuple aliases specialized into positive intersections without hanging:

from collections.abc import Iterable
from typing import Any, assert_type
from ty_extensions import Intersection

type WithIterable[T] = Intersection[Iterable[Any], T]
type A = tuple[WithIterable[A], ...]

def f(x: A):
    reveal_type(x)  # tuple[WithIterable[A], ...]
    assert_type(x[0], WithIterable[A])

Previously, mapping the intersection unfolded A again after its mapping had completed. Each reconstruction could introduce another copy of the tuple, so even revealing x would not terminate.

We extend the existing alias-preservation mode to positive elements. The builder retains aliases to non-object nominal instances on the positive side and continues to apply its sign-specific rules for negated aliases. Aliases that expose object or set operations still expand, and ordinary intersection redundancy checks still run. The regressions cover both alias syntaxes, the returned element type, and an invalid assignment.

[astral-sh-bot]

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 98.24%. The percentage of expected errors that received a diagnostic held steady at 98.24%. The number of fully passing files held steady at 134/146.

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[astral-sh-bot]

ecosystem-analyzer results

No diagnostic changes detected ✅

Large timing changes:

Project	Old Time	New Time	Change
more-itertools	0.13s	0.21s	+60%

Full report with detailed diff (timing results)