[ty] Correct enum alias detection and scalar constructors

[charliermarsh]

Summary

Enum member metadata currently uses full Type values when identifying duplicate-value aliases. Literal metadata such as promotability is therefore treated as part of runtime identity, so equivalent values can incorrectly remain separate canonical members.

We also only look for value-transforming __new__ implementations on enum classes. Python's EnumType can instead use a user-defined __new__ from a non-enum data-type mixin to construct the member's scalar payload. Ignoring that constructor can leave .value as the declared literal, skip constructor argument validation, and mark a runtime-reachable equality branch as unreachable.

This keys alias detection by runtime literal kind and payload, and includes user-defined data-type mixin constructors in enum value-construction metadata:

from enum import Enum

class OffsetInt(int):
    def __new__(cls, value: int) -> "OffsetInt":
        return int.__new__(cls, value + 1)

class Offset(OffsetInt, Enum):
    VALUE = 1

reveal_type(Offset.VALUE.value)  # Any

Enum aliases now resolve to the same canonical member even when their inferred literal types differ only in metadata, while transformed scalar-mixin values remain conservative unless we can model the constructor.

[astral-sh-bot]

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 94.47%. The percentage of expected errors that received a diagnostic held steady at 89.19%. The number of fully passing files held steady at 95/134.

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[astral-sh-bot]

ecosystem-analyzer results

No diagnostic changes detected ✅

Flaky changes detected. This PR summary excludes flaky changes; see the HTML report for details.

Full report with detailed diff (timing results)